The U.S. Army Corps of Engineers said Sunday that it would not approve permits to build the Dakota Access oil pipeline under Lake Oahe, a reservoir on the Missouri River near the Standing Rock Sioux reservation. The decision halted construction, a major victory for members of the tribe and other activists who have been protesting for months to block the project. Opponents of the $3.8 billion, 1,172-mile project say it would threaten the tribe’s water supply and sacred Native American sites. The Army said alternative routes would have to be explored. Protesters said they intended to defy an order to leave their camp by Monday, and authorities said they would not forcibly move anyone.