The Montana Supreme Court reinstated a complaint brought by two oil companies against a mineral company after a lower court improperly found it was entitled to sovereign immunity as an arm of the Assiniboine and Sioux Tribes.

While stopping short of adopting a bright-line rule of barring an entity incorporated under state law from claiming tribal sovereign immunity, the majority agreed with Lustre Oil Co. and Erehwon Oil & Gas’ contention that the lower court did not properly weigh relevant jurisdictional factors when it concluded that A&S Mineral Development Co. was entitled to sovereign immunity.