Let’s put this into perspective. Secretary of Defense James Mattis authorized the deployment of 5,200 active duty U.S. troops to the border of Mexico to help deal with a walking caravan of civilians fleeing Central American violence in pursuit of asylum in the United States. That means there will soon be more than twice as many U.S. active duty military on the southwest border than fighting ISIS in Syria and approximately equivalent to the number of U.S. active duty troops in Iraq.

So why do President Trump and Mattis view this migrant caravan of fewer than 4,000 civilian refugees, one thousand miles and at least six weeks away from the border, to be a more significant threat, much less an appropriate priority, for an already overstretched U.S. military. I’m sure they don’t. It is not a threat. This is a craven political stunt by President Trump ahead of the U.S. midterm elections, and a cynical capitulation by a secretary of defense who has prided himself on improving the readiness, focus, and lethality of the U.S. armed forces and who was once known as a no-nonsense warfighter.

If Mattis does not believe the migrants are a threat that warrants tasking 5,000 active troops to the border, he should say so and resign. If he does believe it, he should explain why, in detail before Congress, immediately. If Republicans in Congress won’t put Mattis in the witness chair, he should walk to the Pentagon briefing room and do it himself.

The Department of Defense’s active participation in such an obvious stunt sets a dangerous precedent and abuse of our military. It will undermine the president’s credibility with the American public the next time circumstances require the deployment of U.S. troops to address a real threat to the U.S. homeland.

This deployment already is a named operation – Operation Faithful Patriot – but what exactly is their mission? The Posse Comitatus Act, an intentional limitation on presidential authority, restricts active duty military from performing law enforcement operations inside the United States. They cannot enforce federal or state laws. They cannot detain non-combatants at the border. So the president and Mattis are sending them to do what exactly? According to Gen. Terrence O’Shaughnessy, commander of U.S. Northern Command, these active troops will be providing “logistical support” and “hardening the border.” So we are sending active duty troops to put up razor wire and give helicopter rides to the border police? Why not just deploy additional National Guard troops who are more than capable of performing these duties and whose involvement does not raise all the thorny legal questions? The answer, again, is that Trump just wants to drive up the public perception of an invasion threat that does not exist.

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Setting aside the questionable mission rationale for deploying active duty military to the border, it is also a colossal waste of taxpayer money. Worse, it has been reported that the Department of Homeland Security will not reimburse the Department of Defense for the costs associated with this deployment, meaning that DOD will likely need to find ways to offset this unfunded mandate. Depending on the length of the deployment, these costs could be significant. When President George W. Bush sent National Guard troops to the border for Operation Jump Start in 2006, it cost approximately $1.2 billion over two years.

Mattis should be prepared to face serious questions from members of Congress about why this mission requires double the active duty troops, what exactly the mission and authority structure of these active duty troops will be, how long they will be deployed, how the Pentagon will pay for this deployment and what other national security priorities will be affected by this choice. This is a deliberate misuse of our hard-working active duty military and arguably an abuse of power by the President. Surely, the late Arizona Senator John McCain would be posing those tough questions. This Congress should do the same. The American people and the U.S. military deserve answers.

Kelly Magsamen is vice president for national security and international policy at the Center for American Progress. She previously served as a senior official in the U.S. Department of Defense and on the National Security Staff of both President George W. Bush and President Obama.