The Standing Rock Sioux Tribe won a major victory on Sunday in its battle to block an oil pipeline being built near its reservation when the Department of the Army announced that it would not allow the pipeline to be drilled under a dammed section of the Missouri River.

The Army said it would look for alternative routes for the $3.7 billion Dakota Access pipeline. Construction of the route a half-mile from the Standing Rock Sioux reservation has become a global flash point for environmental and indigenous activism, drawing thousands of people out here to a sprawling prairie camp of tents, tepees and yurts.

“The best way to complete that work responsibly and expeditiously is to explore alternate routes for the pipeline crossing,” Jo-Ellen Darcy, the Army’s assistant secretary for civil works, said in a statement. The move could presage a lengthy environmental review that has the potential to block the pipeline’s construction for months or years.

But it was unclear how durable the government’s decision would be. Sunday’s announcement came in the dwindling days of the Obama administration, which revealed in November that the Army Corps of Engineers was considering an alternative route. The Corps of Engineers is part of the Department of the Army.

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