Although Secretary of Defense Jim Mattis has been regarded as the “adult in the room” in President Trump’s cabinet, the only way he can preserve his honor and the honor of U.S. military service personnel is to resign his post. After Mattis’s recent trip to the U.S.-Mexico border to inspect troops sent there by the President—in a pre-midterm election stunt to purportedly defend against an insidious immigrant caravan that allegedly contained Middle Eastern terrorists and hardened criminals—Mattis has egg on his face for lending his formidable reputation to help justify this misuse of active-duty U.S. forces.

Even Mattis had to stammer out an evasive answer when asked by a skeptical soldier to explain the goal of the troops’ mission: “Short term, get the obstacles in. Long term, it’s somewhat to be determined.”

He also said a bit condescendingly that Secretary of Homeland Security Kirstjen Nielsen’s Border Patrol would “do all the work, but we’re standing behind them as a confidence builder, and that sort of thing.”

Yet the Border Patrol routinely deals with groups of immigrants trying to get across the border and hardly needs any hand-holding by a military untrained and inexperienced in such a mission. In fact, the Posse Comitatus Act of 1878 prohibits the military from domestic law enforcement, so they are severely constrained in the help they can provide the Border Patrol.

Many military personnel will likely eventually sour on Trump, if they haven’t already. Although Trump has launched attacks on former military heroes—such as John McCain and Admiral William H. McRaven, who commanded the mission that successfully killed Osama Bin Laden during the Obama administration—and failed to honor fallen military personnel at Arlington Cemetery on Veterans Day and skipped a similar ceremony in France due to the weather, Trump’s support in the military has been shored up by slathering the armed services with wads of cash during the last two years.

Because Trump’s massive business tax cut without cutting other government spending has blown a big hole in the federal budget deficit—thus further adding to the already gargantuan national debt of almost $22 trillion—that financial largesse for the military is at an end. And such fiscal irresponsibility will likely swamp even a congressional commission’s report that makes the ludicrous argument that despite the U.S. spending four times what the Chinese expend on defense and 10 times what the Russians spend, the U.S. risks losing a war with these countries.

No matter that this vast disparity of spending has accumulated over decades now. If the commission’s findings were really true—instead of merely being a political ploy to attempt to stanch the likely reversal of the Pentagon’s recent funding splurge—the American taxpayer should ask where all the trillions spent on defense during those decades have been wasted.

Unfortunately, the Pentagon couldn’t come up with an answer. Under Mattis’s guidance, most of the big agencies of the Department of Defense failed the first ever audit of that massive and slothful bureaucracy.

Yet as bad as this situation is, the reason Mattis should step down is that he has shilled for the President’s abuse of active-duty military forces for a political purpose. It would have been bad enough had Trump called in all National Guard forces, which have traditionally had at least some role in meeting contingencies here at home; the bulk of the forces sent to the border were active-duty forces. Such forces should be used to defend the country from legitimate security threats, not used to score points with a president’s political base before an election, especially when civilian authorities could handle any such “problem” (if it were legitimate) easily.

One of the biggest apprehensions of the Constitution’s framers at the nation’s founding, which has lingered throughout much of the nation’s history, was the danger that a standing army posed to liberty. This concern becomes especially acute when a commander in chief, with no sense of constitutional or institutional norms, begins using the active forces for political purposes, and the cabinet official in charge of the U.S. armed forces—a former general in that military, no less—not only fails to resign but actually shills for that politician. A Mattis resignation might make Trump think twice before again flagrantly misusing active military forces.