MURPHY, Circuit Judge.

These two cases challenge the jurisdiction of the State of South Dakota to impose its motor vehicle excise tax and registration fee on Indians who live within the boundaries of a reservation. In one case, the United States sued for declarative, injunctive, and compensatory relief on behalf of the Cheyenne River Sioux Tribe and its members. The second case involves claims brought by the Rosebud Sioux Tribe for equitable relief.

South Dakota Codified Laws section 32-5B-1 provides that residents shall pay an excise tax on the value of any motor vehicle purchased or acquired for use in the state and required to be registered. S.D. Codified Laws Ann. Section(s) 32-5B-1. The excise tax is a one time assessment collected by the county in which the owner resides when the vehicle is first licensed in the state. Id. Section(s) 32-5B-10. Payment of the excise tax is required for the issuance or transfer of state vehicle title, id. Section(s) 32-5B-14, and is thus a condition precedent to registration and issuance of state license plates. The proceeds are allocated to the state highway fund. Id. Section(s) 32-5B-17. Failure to pay the excise tax is a misdemeanor. Section(s) 35-5B-1.

South Dakota Codified Laws section 32-5-5 imposes a separate motor vehicle registration fee on state residents. S.D. Codified Laws Ann. 32-5-5. The annual fee is based on the weight of the vehicle and ranges from $20 to $40 for average noncommercial vehicles. It is collected at the time the owner obtains license plates or renewal tags, and is a condition precedent for their issuance. Id. A percentage of the collected fees goes toward administrative costs, and the remainder is allocated to various road funds. Failure to pay the registration fee is a misdemeanor. Section(s) 32-5-2.4.

I.

The Cheyenne River Sioux Tribe is a federally recognized Indian tribe. In 1868 the Fort Laramie Treaty, 15 Stat. 635, established the Great Sioux Reservation for the use and occupancy of the Sioux Nation. Congress later created the Cheyenne River Indian Reservation in 1889 on part of the treaty land as a separate reservation for the Cheyenne River Sioux Tribe. See South Dakota v. Bourland, 508 U.S. 679 (1993). In 1908 Congress opened a significant portion of the reservation to non-Indian settlement, but this did not diminish the reservation. See Solem v. Bartlett, 465 U.S. 463 (1984). The reservation wholly encompasses Dewey and Ziebach counties in the State of South Dakota. Its residents include tribal members, nonmember Indians, and non-Indians.

The tribe believes that all Indians residing on the Cheyenne River Indian Reservation are immune from state taxation of their motor vehicles, including both the excise tax and the annual registration fee. The tribal council has consistently maintained that the state lacks authority to collect the excise tax and registration fee from tribal members, and there is evidence in the record that at least some members have paid the excise tax and registration fee under protest.

The statutes do not create an exemption for Indian-owned vehicles that are driven exclusively on reservation land, but the state apparently does not enforce its motor vehicle registration laws in such circumstances. The owner of any vehicle driven outside the reservation is subject to criminal penalties for improper registration, however. At one time, tribal law incorporated state traffic laws and required all motor vehicles driven on the reservation to have valid state license plates. In 1994 the tribe enacted its own motor vehicle registration system, but has not yet implemented it to avoid imposing double fees on reservation residents.

On September 3, 1992 the United States brought suit on behalf of the Cheyenne River Sioux Tribe and its members, seeking a declaration that the state lacks jurisdiction to impose its motor vehicle excise tax and registration fee on Indians residing on the Cheyenne River reservation. It also sought an injunction against the collection of the fees and taxes, and monetary damages in the amount of taxes paid between 1986 and the present. On August 24, 1993, the Cheyenne River Sioux Tribe was permitted to intervene as a plaintiff pursuant to Federal Rule of Civil Procedure 24(b).

In February 1995 the district court ruled on cross motions for summary judgment. It held that the excise tax was essentially a personal property tax and thus could not be imposed on tribal members living on the reservation. See Oklahoma Tax Comm'n v. Sac and Fox Nation, 508 U.S. 114, 127-28 (1993). It viewed the annual registration fee differently, concluding that it was a nondiscriminatory fee that could validly be collected from tribal members residing on the reservation who elected to purchase state license plates. The district court did not extend immunity from the excise tax to nonmember Indians and did not award damages to tribal members. Judgment was entered on February 23, 1995, and the parties filed cross appeals.

The United States and the tribe argue that the registration fee operates as a tax and thus cannot be imposed on reservation Indians. They also claim that monetary damages should be awarded for taxes previously paid. The tribe also argues that tax immunity should be extended to nonmember Indians residing on the Cheyenne River reservation. South Dakota responds that the district court correctly determined the registration fee to be valid, denied monetary damages, and held that nonmember Indians were not immune from taxation. It argues on cross appeal that the excise tax is a valid sales and use tax.

A.

As a general rule, a state lacks jurisdiction to tax the lands, activities, and property of tribal members "'within the boundaries of the reservation,'" unless there has been a "'cession of jurisdiction or other federal statutes permitting it.'" California v. Cabazon Band of Mission Indians, 480 U.S. 202, 215 n. 17 (1987) (quoting Mescalero Apache Tribe v. Jones, 411 U.S. 145, 148 (1973)). In other words, a tribal member's on-reservation activities are immune from state taxation absent express congressional authorization of the tax. McClanahan v. State Tax Comm'n of Arizona, 411 U.S. 164, 171 (1973). This is because a state's authority to tax on-reservation activities is limited or preempted by the terms of treaties which set aside reservations for the exclusive use of Indian tribes and by various federal statutes defining the limits of state power. Id. The doctrine of Indian sovereignty reflects the "'deeply rooted'" historical policy of "'leaving Indians free from state jurisdiction and control.'" Id. at 168 (quoting Rice v. Olson, 324 U.S. 786, 789 (1945)). That doctrine "provides a backdrop against which the applicable treaties and statutes must be read." Id. at 172.

Congress has not specifically authorized the taxation of a tribal member's personal property, and South Dakota thus lacks jurisdiction to impose such a tax. This would include any tax or fee that operates as a personal property tax on motor vehicles owned by tribal members living on the reservation. Oklahoma Tax Comm'n v. Sac and Fox Nation, 508 U.S. 114, 127-28 (1993); Washington v. Confederated Tribes of Colville Indian Reservation, 447 U.S. 134, 163 (1980); Moe v. Confederated Salish and Kootenai Tribes of Flathead Reservation, 425 U.S. 463, 480-81 (1976).

The general rule of tax immunity applies to activities that occur on the reservation, including ownership of property, but "different considerations" apply to off-reservation activities. Mescalero, 411 U.S. at 148. Indians who go beyond reservation boundaries are generally "held subject to nondiscriminatory state law otherwise applicable to all citizens." Id. at 148-49. This reasoning suggests that states may impose on tribal members a sales tax or other nondiscriminatory tax on off-reservation purchases. See Tunica-Biloxi Tribe v. Louisiana, 964 F.2d 1536 (5th Cir. 1992).

The parties do not disagree about the general rule of tax immunity for tribal members within the boundaries of a reservation or the authority of the state to tax off-reservation sales transactions. South Dakota admits that it would lack authority to impose state property taxes on members of the Cheyenne River Sioux Tribe living on the reservation, and the tribe concedes that its members could validly be required to pay a sales tax on off-reservation purchases, a properly limited state road use tax, or certain non-discriminatory fees. The dispute here is one of classification: what sort of tax or fee is created by these statutes?

South Dakota argues that the excise tax is a sales tax on off-reservation purchases of motor vehicles. The statute itself speaks of levying an excise tax, "in lieu of" any sales or use tax, on the value of any motor vehicle "purchased or acquired for use" in the state. 32-5B-1. Unlike the state sales tax, the excise tax is not imposed on the retailer or at the time of sale. Compare S.D. Codified Laws Ann. Section(s) 10-45-2 (retail sales tax) with Section(s) 32-5B-10 (excise tax). Nor is the excise tax revenue credited to the general revenue fund. Compare S.D. Codified Laws Ann. Section(s) 10-46-48 with Section(s) 32-5B-17. Although the amount of the tax is based on the "purchase price," that term is defined to include either the actual consideration paid or the actual value of the vehicle. See S.D. Codified Laws Ann. Section(s) 32-5B-4(3)-(5), 32-5B-11.

The excise tax does not operate like a sales tax. It might resemble a sales tax in some cases, such as when a new or used car is purchased from a licensed dealer in South Dakota and registered there by the new owner. In other circumstances, however, the tax more obviously relates to the ownership of the vehicle as property rather than the sales transaction itself. For example, the tax does not apply to all motor vehicle sales in the state, but only to those resulting in ownership by a South Dakota resident. The tax is not paid unless the owner applies for the issue or transfer of South Dakota title. Section(s) 32-5B-14. Conversely, the excise tax does apply to vehicles purchased out of state by South Dakota residents. Moreover, the tax applies to motor vehicles that are being newly brought into the state, but have not recently been purchased. A person moving to South Dakota and applying for a state motor vehicle title is taxed at a percentage of the retail value of the vehicle on the day it enters the state, unless the owner provides sufficient proof that the vehicle was subject to an equal or greater tax in another state. Section(s) 32-5B-11.

South Dakota asserts that Tunica-Biloxi Tribe v. Louisiana, 964 F.2d 1536 (5th Cir. 1992), supports its claim that the excise tax is a valid sales tax, but the tax at issue in that case was different from the South Dakota excise tax. Tunica involved a Louisiana tax on the "sale at retail" of any item of property within the state. That tax did not apply to items purchased out of state. Purchasers from outside Louisiana could also be subject to the sales tax unlike the South Dakota tax which is not applied to out-of-state purchasers. Since the characteristics of the South Dakota tax differ from the Louisiana tax, the outcome here is not inconsistent with Tunica. It is simply wrong to suggest that our decision will create a split among the circuits.

The South Dakota excise tax resembles the personal property taxes found to be preempted in Moe, Colville, and Sac and Fox. Colville involved a Washington "excise tax" which was assessed annually at a percentage of a motor vehicle's value for the "privilege" of using a motor vehicle in the state. 447 U.S. at 162. The Supreme Court held that the tax was similar in all but name to the personal property tax on motor vehicles that had been invalidated as applied to tribal members in Moe, 425 U.S. at 480-81. Colville, 447 U.S. at 163. Similarly, in Sac and Fox, the Supreme Court held that Oklahoma's motor vehicle excise tax and its yearly vehicle registration fee operated as personal property taxes and thus could not be imposed on tribal members living on the reservation. 508 U.S. at 127-28. The South Dakota tax is not identical to the taxes in Moe, Colville, and Sac and Fox, but it has important similarities. Like them, the South Dakota tax is applied to motor vehicle owners, rather than simply to purchasers, and is based on a percentage of the value of the motor vehicle. Sac and Fox, 508 U.S. at 119; Colville, 447 U.S. at 163 (discussing Moe). It is true that this tax is not applied annually, but neither was the tax found invalid in Sac and Fox. Sac and Fox, 508 U.S. at 126.

The South Dakota excise tax essentially operates as a tax on the ownership of a motor vehicle, rather than as a tax on off-reservation sales transactions. The ownership of a vehicle is the sort of on-reservation activity that a state is not permitted to tax without express Congressional authorization. See Sac and Fox, 508 U.S. at 127. The district court did not err in ruling that the state lacks jurisdiction to impose the excise tax on tribal members residing on the reservation.

B.

The United States and the Cheyenne River Sioux Tribe argue that the state similarly lacks jurisdiction to impose its annual registration fee on reservation Indians because it operates to tax on-reservation activities. South Dakota responds that it has authority to impose the fee because it is a nondiscriminatory fee for off-reservation activities. The issue is again one of classification.

The United States and the tribe claim that the fee actually operates as a tax on the ownership of vehicles and is therefore preempted under Sac and Fox, Colville, and Moe. In Sac and Fox the Court held Washington's annual registration fee to be invalid as applied to reservation Indians, but that fee was based on the value of a vehicle and thus was like a personal property tax. Id. at 127-28. It was found to be no different from the taxes held preempted in Moe and Colville. South Dakota's registration fee does not resemble a property tax, however. It is not based on the value of the vehicle, but is a flat fee based on weight, and any proceeds resulting from it are dedicated to highway purposes.

The United States and the tribe also suggest that the fee is a tax on the use of state roads. They concede that a state might have jurisdiction to impose a tax on reservation Indians for the use of off-reservation roads, but argue that the fee here is not "tailored to the actual amount of off-reservation use" as required by Colville, 447 U.S. at 163-64.

In Colville, the Supreme Court rejected the State of Washington's claim that its excise tax was actually a tax on the use of the vehicle within the state. It held that the tax effectively functioned as a personal property tax, which had previously been held invalid as applied to tribal members in Moe:

We do not think Moe and McClanahan can be this easily circumvented. While Washington may well be free to levy a tax on the use outside the reservation of Indian-owned vehicles, it may not under that rubric accomplish what Moe held was prohibited. Had Washington tailored its tax to the amount of actual off-reservation use, or otherwise varied something more than mere nomenclature, this might be a different case.

Id. There is no suggestion in the South Dakota statute that the purpose of the fee is to tax the use of the state roads. Even if it were, the tax is significantly different from a property tax so that the concerns expressed in Colville are avoided.

The registration fee is a nondiscriminatory fee for the registration of a vehicle in the state and the issuance of state license plates. See Moe, 425 U.S. at 469 (Montana fee required for registration and issuance of state license plates could be imposed on reservation Indians). It is not collected from owners of motor vehicles that are not registered with the state, including those driven exclusively within the boundaries of the reservation. The concerns related to state taxation of on-reservation activities are not present here because the fee is not based on any on-reservation activity. Accordingly, the district court did not err in ruling that Indians who elect to purchase South Dakota license plates or renewal tags can be required to pay the annual registration fee.

C.

The Cheyenne River Sioux Tribe argues that immunity from the excise tax should not be limited to tribal members but should extend to nonmember Indians residing on the reservation. South Dakota responds that the interests of nonmember Indians have not been properly raised, and argues that nonmember Indians are subject to state taxation because they have the same status as non-Indians.

The state's power to tax nonmember Indians is not directly raised by this action, because no nonmember Indians are parties to this case. The United States initiated the action "on behalf of the Cheyenne River Sioux Tribe and its members," and the tribe intervened "to protect its sovereign right to raise revenue to finance its state government . . . ."

Even if the issue had been properly presented, the arguments made are not convincing. As discussed above, the tax immunity of reservation Indians is premised on the preemption of state laws by treaty and statute and informed by notions of tribal self government. These reasons underlying immunity do not have the same force as applied to Indians who are not members of the governing tribe. See Colville, 447 U.S. at 160-61 (retail sales tax immunity for on-reservation sales does not extend to nonmember Indians residing on reservation).

The tribe argues that the interest in tribal self government extends to nonmember Indians because Congress has defined tribal self government to include all Indians. Although Congress has defined tribal powers of self government to include criminal jurisdiction over "all Indians," 25 U.S.C. §(s) 1301, there is no question that South Dakota retains civil regulatory jurisdiction over nonmember Indians in the same way that it does over non-Indians on the reservation. See id.

The tribe also asserts that the terms of the 1868 treaty creating the Great Sioux Reservation establishes that nonmember Indians have an interest in tribal self government. It points to language setting aside land for the use of the Sioux Nation and "for such other friendly tribes or individual Indians as from time to time they may be willing . . . to admit amongst them." Art. 2. ( 15 Stat. 653). Nonmember Indians living on the reservation are not in fact involved in tribal self government, however. The Cheyenne River Sioux Tribe's constitution and bylaws do not allow nonmember Indians to vote in tribal elections or hold office on the reservation.

The tribe also argues that at a minimum any Sioux Indian residing on its reservation should be immune from taxation because all Sioux are part of the Sioux Nation. The Great Sioux Reservation was divided into separate reservations for various Sioux tribes, including the Cheyenne River Sioux. Although the different Sioux tribes share common ties, they exist as separate entities with their own constitutions and governing structures. Nonmember Sioux residing on the Cheyenne River reservation are not allowed to vote or hold office, and are not actually constituents of the governing tribe.

D.

The United States and the tribe argue that monetary damages should be awarded to compensate for past payments of the excise taxes by tribal members. The district court rejected the claim for damages because previous cases addressing motor vehicle taxation had not been applied retroactively. South Dakota offers several alternative rationales for denying monetary relief.

The district court correctly noted that the opinions in Moe, Colville, and Sac and Fox did not grant retroactive relief, but those cases did not involve claims for monetary damages. Rather they were claims for declaratory and injunctive relief. When a state tax is declared to be invalid either "because . . . it [is] beyond the State's power to impose" or "because the taxpayers were absolutely immune from the tax," the State must "'undo' the unlawful deprivation by refunding the tax previously paid under duress." McKesson Corp. v. Division of Alcoholic Beverages and Tobacco, 496 U.S. 18, 39 (1990) (citing Ward v. Love County Board of Comm'rs, 253 U.S. 17, 24 (1920)). In other words, the holding in such cases would be applied retroactively.

The state claims that a damage award is barred by the Eleventh Amendment. It cites Blatchford v. Native Village of Noatak, 501 U.S. 775 (1991), in which the Supreme Court held that the Eleventh Amendment would bar damage claims brought by Indian tribes against a state. The Eleventh Amendment does not bar suits brought by the United States on behalf of Indian tribes or their members, however. United States v. Minnesota, 270 U.S. 181, 195 (1926); United States v. Board of Comm'rs of Osage County, 251 U.S. 128, 133 (1919). The Blatchford Court recognized that the tribal claims would not have been barred if brought by the United States, but held that tribal access to federal court was not as broad. Blatchford, 501 U.S. at 783-84. The Eleventh Amendment does not apply in the Cheyenne River case because the United States brought the action.

The state also argues that damages should be denied because payment of the taxes was required by tribal law and thus was not coerced by the state. A 1987 resolution incorporated into tribal law certain provisions of the South Dakota Traffic Code, including the requirement that vehicles bear valid state license plates. Neither that resolution nor a clarifying resolution passed in 1990 addressed the payment of the state excise tax or suggested that its collection from tribal members was valid. The tribal council has consistently protested the imposition of state motor vehicle taxes on its members, and has now adopted its own registration system.

An additional argument by the state is that the excise taxes were paid voluntarily and thus cannot be recovered back as a matter of law or equity. Taxes that are voluntarily paid because of a mistake of law cannot be recovered back, see Security National Bank of Watertown v. Young, 55 F.2d 616, 619 (8th Cir.), cert. denied, 286 U.S. 551 (1932), but taxes paid under duress or coercion are recoverable, and state refund procedures do not limit such recovery. McKesson, 496 U.S. at 31; Board of Comm'rs of Jackson County v. United States, 308 U.S. 343, 350 (1939) (citing Osage County, 251 U.S. 128 and Ward, 253 U.S. 17).

The record related to the nature and amount of the damage claims in the Cheyenne River case has not been fully developed. There is anecdotal evidence describing informal protests by several tribal members at the time of payment, claiming that the tax was paid to avoid criminal penalties, and suggesting that the state refund procedure is inadequate. These issues were not reached by the district court because of the mistaken view that damages were necessarily barred. The judgment should be reversed to the extent that it eliminated any possibility of damages. On remand the district court should determine whether damages should be awarded and, if so, in what amount.

II.

The Rosebud Sioux Tribe is also a federally recognized Indian tribe, and the initial history of the Rosebud Indian Reservation parallels that of the Cheyenne River reservation. Initially part of the Great Sioux Reservation established by the Fort Laramie Treaty, 15 Stat. 635, the Rosebud reservation was set apart by the Act of March 2, 1889, 25 Stat. 888, 892. The reservation was diminished by Congressional acts near the turn of the century, and now consists of Todd County, South Dakota. See Rosebud Sioux Tribe v. Kneip, 430 U.S. 584, 615 (1977). Tribal members living on allotted lands outside the reservation are considered to be in Indian country. Id. at 615 n. 48.

On October 3, 1994, the Rosebud Sioux Tribe brought suit in federal district court against the South Dakota Secretary of Revenue for declaratory and injunctive relief against the imposition of state motor vehicle excise taxes and registration fees on Indians residing on the reservation. The case was assigned to the same judge before whom the Cheyenne River action was pending. The Secretary filed a motion to dismiss, and the tribe moved for a preliminary injunction.

On February 23, 1995, the same day that judgment was entered in the Cheyenne River case, the district court held a hearing on the motions in the Rosebud case. On May 23, 1995 it issued an opinion and order denying the Secretary's motion to dismiss, partially granting and partially denying the tribe's request for a preliminary injunction, and transferring the case to another judge. The opinion and order was short, and the court did not make detailed findings or discuss the basis for its disposition.

The district court's opinion recognized that the issues were similar to those raised in the Cheyenne River case. It granted a preliminary injunction against the collection of the motor vehicle registration fee from tribal members who drive only within the undiminished portion of the Rosebud reservation, but denied the tribe's motion in all other respects. It did not specifically enjoin the collection of the excise tax from tribal members living on the reservation, and the tribe filed this interlocutory appeal from that partial denial of its motion.

The Rosebud tribe argues that the reasoning in the Cheyenne River case should apply to Indians residing on the Rosebud reservation and that the district court erred by failing to enter an injunction against the collection of excise taxes. The tribe argues that the order is inconsistent with the Cheyenne River judgment because it did not enjoin the state from continuing to collect excise taxes from members of the Rosebud tribe living on the reservation. The Secretary responds that the district court did not abuse its discretion when it partially denied the motion for a preliminary injunction and that the merits of the underlying action are not an issue on appeal.

The issue on this interlocutory appeal is whether the district court abused its discretion or committed an error of law. See Dakota Indus., Inc. v. Dakota Sportswear, Inc., 988 F.2d 61, 63 (8th Cir. 1993). Although the order would be easier to review if it contained more detailed findings and reasoning, it appears that the earlier ruling in the Cheyenne River case was a major factor in the court's thinking. The same judge had entered a declaratory judgment in that case three months before, holding that South Dakota could not impose its motor vehicle excise tax on tribal members living on the reservation. In its opinion in that case, the district court denied injunctive relief since there was "no reason to believe the defendants will not comply with the requirements of this decision." Implicit in its Rosebud ruling is a similar assumption that the state was complying with the requirements of the Cheyenne River decision and had stopped collecting excise taxes from tribal members living on reservations. Injunctive relief is unnecessary where there is no showing of irreparable harm, and nothing in the record suggests that South Dakota continues to collect the excise tax from tribal members residing on Indian reservations. The tribe has not shown that the grant of only partial relief was an abuse of discretion or clearly erroneous.

III.

In conclusion, the judgment entered in the Cheyenne River case is affirmed in part and reversed in part and remanded for consideration of the remaining damages issues. The order granting partial injunctive relief in the Rosebud case is affirmed.