BISMARCK, N.D. — The developer of the stalled Dakota Access oil pipeline could get clearance from the Army as early as today to finish the $3.8 billion project, though the American Indian tribe that has led the battle against the project for months has vowed to continue the fight.

The Army said Tuesday that it will allow the four-state pipeline to cross under a Missouri River reservoir in North Dakota, the last big chunk of construction. The Standing Rock Sioux promised to fight the development in court.

The Army intends to cancel further environmental study and allow the Lake Oahe crossing, according to court documents the Justice Department filed that include letters to members of Congress from Deputy Assistant Army Secretary Paul Cramer.

The 1,200-mile pipeline would carry North Dakota oil through the Dakotas and Iowa to a shipping point in Illinois. Dallas-based developer Energy Transfer Partners had hoped to have the pipeline operating by the end of 2016, but construction has been stalled while the Army Corps of Engineers and the company battled in court.

The Standing Rock Sioux, whose reservation is just downstream from the crossing, fears a leak would pollute its water. The tribe has led protests that drew hundreds and, at times, thousands of people who dubbed themselves “water protectors” to an encampment near the crossing. ETP said the pipeline will be safe.

Details of the tribe’s legal challenge were still being worked out, attorney Jan Hasselman said Tuesday. But tribal Chairman Dave Archambault said the tribe is “undaunted” by the Army’s decision. Even if the pipeline is finished and begins operating, he said, the tribe will push to get it shut down.