A car drives past the new bollard-style U.S.-Mexico border fencing in Santa Teresa, N.M., as pictured from Ascension, Mexico, on Aug. 28. (Jose Luis Gonzalez/Reuters)

On Tuesday, Defense Secretary Mark T. Esper notified members of Congress that he would take $3.6 billion from military construction projects to build 175 more miles of wall along the U.S. border with Mexico. He can do that legally only because President Trump in February declared an emergency along the border — which, he acknowledged, he did to circumvent the fact that Congress had allocated only a token amount to build the wall, much less than he requested.

This isn’t the first chunk of money that the Defense Department has shifted to build the wall. Earlier this year, it used $2.5 billion that Congress had allocated to stop drug trafficking to expand the wall; Trump argues that the border wall will accomplish that goal. The new redistribution of funds brings to $6.1 billion the total amount of Defense Department spending repurposed for the wall. Esper and other Pentagon officials are now urging Congress to “backfill” — or replenish — the funds being taken out of other military construction projects. Democrats have vowed not to do so, since shifting the money clearly circumvents congressional intent.

Shifting funds already allocated for other military construction has a range of political, legal and national security consequences. Here’s how.

The military budget will be further politicized

This latest partisan face-off over the defense budget showcases how the country’s increasing political polarization is challenging the military.

One issue is the separation of powers. Article I of the Constitution gives the power of the purse to the legislative branch. But if the Defense Department can move money against legislators’ wishes and simply ask them to refill those coffers to save priority projects, the executive branch effectively assumes that power.

Then there’s the politicization of the military budget itself. The Pentagon’s budget is usually distributed in both Republican and Democratic districts and for bipartisan priorities — keeping both parties happy. As political scientist Chris Higginbotham put it in his 2017 dissertation on Congress and the defense budget, whether members are “motivated by fear of potential foreign enemies, nationalistic pride, concern for service members, or economic advantage, the defense budget is as close to a bipartisan priority as can be found in U.S. society.”

That changes if a president spends military funds explicitly for partisan political gain. True, Esper appears to be doing his best to distribute the political costs as evenly as possible, along partisan and geographic lines. But the risk is that military spending ceases to involve compromises and bargains among different players, and instead becomes a zero-sum game in which one party uses that budget to achieve political goals — like winning elections — at the opposition party’s expense.

Of course, this trend did not start with Trump. In 2011, Congress and President Barack Obama enacted the 2011 Budget Control Act, which set restrictive caps on defense and domestic spending. That meant that biennial deals have since been required to raise the caps — and risked government and Pentagon shutdowns when lawmakers couldn’t agree.

Political scientist Chuck Cushman argues that the trend stretches back to 1991. When the Cold War ended, he says, so did that era’s consensus over national security policy — leaving members of Congress “freer to inject local concerns into defense.” Whatever its origins, the Trump administration is further politicizing the defense budget.

Border funding will come at the expense of other construction priorities.

The Defense Department has many priorities. The United States is still fighting in Afghanistan, deploying counterterrorism forces to the Middle East and elsewhere, watching tensions rise in East Asia, and working to develop new military capabilities that can counter China and Russia.

Even within the category of military construction — only 3 percent of defense spending — the budget is already stretched. Former secretary of the Navy Ray Mabus tweeted Wednesday: “As hurricane heads for Camp Lejeune Trump takes $3.6b from military for needless wall. Same amt Marines say needed to fix Lejeune after last storm.”

Meanwhile, it may be risky to defund the 127 military construction projects that the Pentagon has identified. Some of the projects involve weapons training and maintenance. European bases will take many of the reductions, as a Washington Post analysis last year suggested and the Pentagon’s list confirms. Doubtless the Pentagon worked to cut in areas that would not undermine operations and readiness. But the cuts could further destabilize the U.S. role in the NATO alliance, as many European nations are already wary of the Trump White House’s calls to divest from European bases.

This move involves the U.S. military in partisan politics

As I wrote here at TMC last year, deploying the military to the U.S.-Mexico border just before the 2018 midterm elections threatened the armed forces’ reputation as a nonpartisan, apolitical institution. This latest move does so again.

Ironically, partisans calling on each other to leave the military out of politics actually heightens the perception that the military is a political constituency. Rep. Debbie Wasserman Schultz (D-Fla.), chair of the military construction and veterans affairs subcommittee of the House Appropriations Committee, called the move “theft from the military.” A group of Democrats on the committee asked Esper to justify “why a border wall is more important to … the well-being of our service members and their families” than the projects now on the chopping block. House Armed Services Committee member Mac Thornberry (R-Tex.) said Democrats’ threats to withhold the backfill money “only forces our troops to pay for political discord in Washington.”

Although these concerns may be entirely sincere, they nevertheless tie the military’s fate to the outcome of partisan disputes.

Alice Hunt Friend (@ahfdc) is a senior fellow at the Center for Strategic and International Studies and a PhD candidate at American University’s School of International Service. She was the principal director for African Affairs in the Office of the Secretary of Defense from 2012-2014.