The U.S. Supreme Court weighed how broadly to read a 19th century treaty that a tribe claims exempts it from a state fuel tax, questioning attorneys on both sides about potential wider tax implications.

Cougar Den, a Yakama-owned business, argued the tribe is exempt from a tax on fuel entering Washington because of the Yakama Treaty of 1855, which includes a provision guaranteeing the tribe’s right to travel on highways. Washington, which assessed Cougar Den millions in taxes...