A dispute over the practice of flaring natural gas from oil wells fuels the legal controversy in this case: the scope of Native American tribal court authority over nonmembers. Several members of the MHA Nation sued numerous non-tribal oil and gas companies in MHA tribal court. Those companies operate oil wells on lands within the Fort Berthold Indian Reservation that have been allotted to individual tribe members but are held in trust by the federal government. The tribe members alleged the companies owed royalties from wastefully-flared gas. Some of these companies unsuccessfully contested the tribal court’s jurisdiction over them in tribal court. Then they initiated this action in federal court to enjoin the tribal court plaintiffs and tribal court judicial officials. The district court issued a preliminary injunction, and the tribal court plaintiffs and officials separately appealed. We affirm the injunction because we conclude suits over oil and gas leases on allotted trust lands are governed by federal law, not tribal law, and the tribal court lacks jurisdiction over the nonmember oil and gas companies.