Does being cozy with the military, because you recently served in it, make someone unfit to lead the military?

That’s the theory behind a decades-old practice of making sure that the person leading the Defense Department is a civilian, or has at least been out of military service for close to a decade. And it’s why there’s some initial hesitation, including among Democrats, about President-elect Joe Biden’s pick to head the Defense Department, retired Army Gen. Lloyd J. Austin III.

There’s a law that says the leader of the Defense Department needs to have at least seven years’ distance between themselves and military service. The law, originally passed in 1947 and originally with a requirement that a candidate be retired from military service for a minimum of 10 years, is derived from the concept that the military should serve civilians, not the other way around. (Congress changed the law in 2007 to a seven-year minimum.)

The roots of this desire to have a civilian head the military run deep. At the outset of the nation, Congress was really worried about how its military could be seized by malign actors who could overthrow their democratic experiment. Founders took pains to put lots of checks and balances on the military, such as congressional reconsideration of defense funding every two years. The big protection was having a civilian control the military.

“This has been a recurring theme in U.S. history, separation of military and civilian authority,” said Mark Cancian, a retired colonel and military expert at the Center for Strategic and International Studies. “It’s written into the Constitution. It’s in the Declaration of Independence. That was one of the complaints we had about the British.”

Having a civilian lead the military is “a fundamental tenet underpinning the design and operation of the American republic since its inception in 1776, if not before,” Kathleen J. McInnis wrote in a 2017 Congressional Research Service report to Congress. “The Founding Fathers believed that subordination of the military to the authority of civil masters was critically important in order to prevent the emergence of a new form of tyranny or dictatorship.”

It’s similar to how the founders didn’t want a lifelong leader in the White House. They took every step to prevent leaders from undermining a government controlled by the people. And they specifically wanted a military leader who responded to the duly elected president and wouldn’t be in cahoots with the military branches over the president’s wishes.

Eventually, Congress put this into law. And the law is so sacrosanct that only twice in the past 73 years has there been a Pentagon leader more recently retired from military service: In 1950 under President Harry S. Truman, and then again in 2017 for Jim Mattis to lead President Trump’s Defense Department.

Now, Biden is going to ask Congress to give him a waiver to allow Austin to serve. Austin is particularly fresh from the military; he’s been retired from the Army for just four years, and most of his experience comes from the field, rather than in Washington. But Biden, in one of the most forceful defenses for one of his nominees yet, urged Congress to confirm Austin anyway by having both chambers approve a waiver for him, saying he was the best candidate for the moment.

“The fact is,” Biden wrote in The Atlantic, “Austin’s many strengths and his intimate knowledge of the Department of Defense and our government are uniquely matched to the challenges and crises we face. He is the person we need in this moment.”

But two exemptions for two presidents in a row is a troublesome trend for many military scholars. “We’re going beyond precedent to a pattern now,” Cancian said, “and that’s a problem because it blurs this line between military and civilian.”

Some in Congress have argued that this law is outdated and restricts whom the president can choose. What if someone more recently retired is the best candidate?

That was the consensus, especially among Republicans who controlled Congress, when Trump picked Mattis. “Very few observers, if any, appear concerned that General Mattis, if appointed to the position of Secretary of Defense, will compromise the long-standing American tradition of ensuring that the military remains subordinate to the authority of civilian leaders,” wrote McInnis in her report about the 1947 law. Congress approved a waiver for Mattis, but a number of Democrats opposed making such a change.

Another argument among those who are less inclined to stick to the letter of the law is: Are we really worried, after all these years later, that the military is suddenly going to usurp the will of the president and thus the people? In 2007, the House of Representatives seriously considering cutting the limit to five years. It would “reduce an outdated prohibition and enable the president to choose from a greater pool of qualified candidates with relevant military expertise,” wrote one of its main proponents, then-GOP congressman Walter B. Jones (N.C.).

Maybe we should be concerned about the military getting too powerful, argue some critics of Biden’s pick. Especially after four years of Trump smashing norms in all areas of democracy.

Cancian points out that Trump would refer to generals as “my generals,” demand personal loyalty and give political stump speeches to the troops.

And a particularly harrowing experience for the military came this summer after law enforcement officers gassed peaceful protesters, then military leaders joined the president as he walked across the cleared park for a photo op. (Recently fired defense secretary Mark T. Esper said he was misled about what he was doing, and the Pentagon’s top general apologized for being part of it.)

After all that, “we need civilian leadership and a return to normalcy,” argues Jim Golby, a military expert and adviser to both Biden and Vice President Pence, in the New York Times.

Most of the opposition in Congress to extending a waiver so Mattis could serve came from Democrats, which could complicate Biden’s attempt to get him confirmed by the Senate. He’ll need both chambers of Congress to approve a waiver. And as my Washington Post colleagues noted, some of the Democrats who did vote for Trump’s waiver said they’d never do it again. “I will not support a waiver for future nominees, nor will I support any effort to water down or repeal the statute in the future,” Sen. Jack Reed (R.I.), the top Democrat on the Armed Services Committee, said at the time.

But Austin has another thing going for him that could help gather Democratic support: He’d make history as the first Black person to lead the military. And already, top members of the influential Congressional Black Caucus are saying they support him, reports Politico.

Plus, like it or not, Cancian says, members of Congress may have boxed themselves into approving waivers for recent generals to head the Defense Department. Mattis’s waver got a bipartisan vote, with Republicans especially saying they didn’t have a problem with it. “It’s going to be impossible for them to make a general argument on principle” to oppose this now that Biden wants it, he said.

This has been updated with Biden’s defense of Austin.