A bipartisan group of House lawmakers who want to stop President Donald Trump from pulling U.S. troops from military conflicts and overseas bases are using a once-unusual tactic that has in the last three years become Congress' go-to tool.

Lawmakers from the Senate and the House have in the last three years sought to use legislation to prevent the president from pulling troops from Afghanistan, Syria, South Korea, and now Germany — unless the administration certifies that a withdrawal will not harm U.S. or allied interests.

It's an unusual reversal for Congress, which historically is far more likely to seek to force a return of American troops, not demand that they remain deployed. According to Danielle Lupton, a Colgate University professor who has tracked foreign policy and defense votes since the late 1970s, "the overwhelming majority of votes related to use of force or troop deployments" have sought to bring troops home or constrain the president from using force. Although there are a few cases where Congress voted to support a deployment, Lupton said, it was typically a friendly legislature acting symbolically to reaffirm the president's foreign policy.