President-elect Joe Biden’s initial Cabinet picks have showcased both diversity and talent, but his selection of retired Army Gen. Lloyd J. Austin III has been greeted with fierce criticism from multiple quarters. If confirmed, Austin, who has a long and distinguished military record, would become the first African American secretary of defense. That’s something to celebrate — but because he only retired from the Army four years ago, Austin’s nomination also raises a host of thorny issues about civil-military relations, and it may lead to substantial opposition from key Democrats.

In 1947, Congress reorganized America’s sprawling military and defense-related organizations. World War II had demonstrated that rivalries and poor coordination between the military services undermined military effectiveness, and the National Security Act of 1947 sought to centralize defense-related decision-making and place a civilian, who would not be affected by loyalties to any one branch, at the helm. To that end, Congress mandated that “the Secretary must be appointed from civilian life by the President.” In case this wasn’t sufficiently clear, the law went on to state that “a person who has within ten years been on active duty as a commissioned officer in a Regular component of the armed services shall not be eligible for appointment.”

Although Congress shortened the 10-year cooling-off period to seven years in 2008, there are still sound reasons to consider it generally unwise for recently retired senior officers to run the Pentagon.

Congress was concerned with the effect of residual service loyalties, and this concern remains valid today. But it’s more than that. For one thing, military culture is strong and unique. Officers are trained and habituated to doing things in particular ways, and the military has processes, assumptions and terminology that aren’t always shared by civilians. Words such as “planning” and “strategy,” for instance, can mean different things to military personnel and to civilians. (To most civilians, for instance “planning” means little more than “thinking about what we may want to do in the future.” To the military, “planning” is a complex process involving scores of experts and multiple painstakingly defined steps.)

Partly for that reason, one of the roles of the secretary of defense is to serve as the top “translator” between the military and civilian political leaders: the secretary has to understand what civilian political leaders want and interpret their preferences in ways that translate into effective action on the part of the military. At the same time, the secretary has be able to translate military constraints and requirements and translate those into language and proposals that make sense to civilian leaders. When recently retired senior officers go more or less straight from a senior military role to a civilian one at the top of the Defense Department, it can be very hard for them to serve as effective translators.

In addition, it’s important that the military be, and remain, nonpartisan in nature — especially after President Trump’s multiple efforts to politicize the military. If senior military officers know they can move rapidly and easily from retirement into positions as senior political appointees, there’s a risk that instead of giving civilian leaders honest, nonpartisan advice, they will start shaping their advice in a manner calculated to advance their future chances of obtaining senior civilian political appointments. (The revolving door between the military and defense contractors may also skew incentives, but in a less obviously partisan way.)

Congress has recognized that from time to time, exceptional circumstances may merit waiving the mandated cooling-off period. Congress has granted such waivers just twice, by passing legislation to temporarily override the prohibition on moving too quickly from the military to the job of defense secretary: In 1950, retired Gen. George C. Marshall was granted a waiver, and in 2017, Congress also voted to allow retired Gen. Jim Mattis to become defense secretary despite his recent retirement. Marshall’s waiver, notes the political scientist Eliot Cohen, was granted “under pressure of emergency”: The Korean War was going badly, the Soviets had just developed atomic weapons and President Harry Truman’s first defense secretary had just been fired for incompetence. Many, like Cohen, argued in 2017 that Mattis’s waiver request was justified for similarly urgent reasons: Given Trump’s aptitude for chaos and his apparent fondness for generals, Mattis might be one of the few people able to prevent “wildly stupid, dangerous, or illegal things from happening.”

But, Cohen notes, “there is no such emergency now” — Biden, unlike Trump, is likely to provide a steady hand at the wheel. And in any case, neither Marshall nor Mattis ended up being an unadulterated success as defense secretary. Mattis did manage to rein in some of Trump’s more foolhardy tendencies, but during his tenure as secretary, Pentagon insiders reported that civilian experts were largely sidelined.

As former undersecretary of defense Eric Edelman told me, “People tend to turn to their rolodex of known quantities for advice and support when they get into these senior jobs. … The problem is if you have been in the military for 40 years, guess who is in your rolodex? Mattis was a failure as SecDef … largely because he insulated [himself] amongst a staff of former Marines and Naval officers.”

During my own time working at the Pentagon from 2009 to 2011, I saw firsthand how easy it was for civil-military misunderstandings to cause tensions and lead to bad policy decisions. And since Mattis resigned in December 2018, conditions at the Defense Department have reportedly deteriorated even more dramatically than during his tenure. Trump has left a whopping 40 percent of Senate-confirmed civilian positions unfilled, and the locus of policymaking has reportedly shifted to the uniformed Joint Staff, leaving the civilian staff decimated and demoralized. The Pentagon is a vast but delicate machine: It cannot run effectively without both military expertise and civilian expertise, and if either is weakened, the whole thing starts getting shakier. Today, as the changing security environment presents the military with a range of novel and boundary-crossing challenges, blurring the lines between traditional military and traditional civilian spheres, diversity of opinion and background within the department is more important than ever.

Austin’s selection also seems an odd choice for Biden. Biden’s team reportedly delayed the announcement of his choice for defense secretary to send the message that Biden planned to elevate diplomacy and de-emphasize the military. Selecting a recently retired general seems to undermine that message, particularly when several other widely respected candidates were on Biden’s shortlist, including Michèle Flournoy, for whom I worked while I was at the Pentagon, and Jeh Johnson, who served as Defense Department general counsel and secretary of homeland security under President Barack Obama. Flournoy would have made history as the first female defense secretary, while Johnson would also have been the first Black defense secretary had he been selected — and, unlike Austin, neither of these other potentially pathbreaking nominees would have required a congressional waiver.

In an indication that he recognizes the troubling issues raised by Austin’s selection, Biden just published an unusual essay in the Atlantic assuring readers: “I respect and believe in the importance of civilian control of our military and in the importance of a strong civil-military working relationship at DoD — as does Austin.”

Perhaps. But Austin’s selection also places congressional Democrats in a difficult position. When Congress voted to grant Mattis a waiver in 2017, many prominent Democrats voted “no.” Now they face the prospect of either voting against Biden’s pick or appearing to change their minds primarily for partisan reasons. “Waiving the law should happen no more than once in a generation,” Sen. Jack Reed (R.I.), a former Army officer and the ranking Democrat on the Senate Armed Services Committee, said in 2017. “Therefore, I will not support a waiver for future nominees.”

With so many members of Congress on the record pledging to oppose any future waiver requests, Biden’s selection forces those members to make an impossible choice: Either vote against their party’s president and against the first Black defense secretary, or reverse their previous stance and look like hypocrites who put party over principle. It’s a no-win situation. As one former Senate staffer told me, the pick will burn up political capital for Biden that he might do better to save for future fights.

Of course, formalistic rules about when retired officers may serve do not in themselves guarantee a healthy civil-military relationship. I have argued in the past that we should look beyond such rules and focus primarily on achieving the normative goals those rules are intended to further, because keeping recently retired officers out of top civilian positions is no guarantee of sound policy. There can be little doubt that the national security community, military and civilian alike will rally around Austin and celebrate his successes if he is confirmed — and perhaps, despite his background, Austin will be able to do what Mattis did not. “The civil-military dynamic has been under great stress these past four years,” Biden said in his Atlantic essay, but Austin “will work tirelessly to get it back on track.” Let’s hope he’s right.