WASHINGTON — Already, President Trump has sent 1,000 military forces to the southwest border in a decision he said will protect Americans from a caravan of migrants coming from Central America. He has pledged to deploy at least 4,000 more troops by week’s end, as part of a last-ditch effort to galvanize the Republican base before the midterm elections on Tuesday.

The troops, the president said, will be “waiting” for the caravan of unarmed men, women and children fleeing violence in Central America. Mr. Trump has also said the caravan may include Islamic terrorists, though he has shown no evidence to back up the claim.

“This is an invasion of our Country and our Military is waiting for you!” Mr. Trump tweeted Monday.

But using the world’s most powerful military to stop some 3,000 unarmed migrants who may or may not make it to the United States border might not be the best use of taxpayer dollars at the Defense Department. Here’s why.

It won’t be cheap.

Defense Department officials say the border deployment is set to last until Dec. 15. It is not clear how much it will cost to send active-duty military troops to the border for the 45-day deployment, and neither the Pentagon nor the White House has provided a price estimate.