First, President Trump drastically ramped up the number of active-duty troops at America’s southwestern border.

Then, as protests over deportations broke out last week in Los Angeles, he dispatched the National Guard to help put them down.

When California’s governor objected, Mr. Trump dug in, sending Marines and even more National Guard troops.

Since taking office in January, Mr. Trump has, step by step, expanded domestic use of the military, testing the legal and political limits on involving troops trained to fight foreign wars in roles traditionally carried out by the local police or Border Patrol.

There are now more U.S. troops deployed to Los Angeles than serving in Syria and Iraq, a fact the chief Pentagon spokesman, Sean Parnell, promoted on social media on Wednesday.

“This is exactly what the American people voted for,” Mr. Parnell wrote. “Defense of our people & our homeland.”