Then-Marine Gen. James Mattis testifies on Capitol Hill in 2013. (Evan Vucci/Associated Press)

Ruben Gallego, a Democrat, represents Arizona’s 7th District in the U.S. House of Representatives.

As a combat veteran of the Marine Corps and a longtime admirer of Gen. James N. Mattis, I was sad to annouce this month that I could not in good conscience support granting him a waiver to serve as our 26th secretary of defense.

I explained that my decision was motivated not by political considerations but by concern for the enduring American principle of civilian control of the military. I lauded Mattis’s eminent qualifications and leadership skills while affirming that this central tenent of our democracy should matter more than any single individual.

What happened next was revealing.

Despite my considered words, I got an earful from Marines across the country, including men I served alongside in Iraq. They called and wrote letters. They tweeted and texted. In some colorful language that I can’t repeat in this space, they questioned my loyalty to the Marine Corps and to our country.

Paradoxically, their passionate defense of Mattis and their anger toward me confirmed my reservations about his appointment. For me, the reaction immediately verified the wisdom of Congress in establishing a cooling-off period for former military leaders. The anger that my stance elicited among many of my fellow Marines demonstrated, albeit on a small scale, the danger to our democracy of a defense secretary coming to power with the ardent loyalty of the men and women he recently commanded.

The members of Congress who, in 1947, enshrined in law this period of separation had fresh memories of World War II. Like our Founding Fathers, they recognized that political leaders should derive their authority from the will of the people — not the personal fealty of members of the armed forces. As a result, they were wary of a decorated general slipping off his uniform and immediately stepping into an ostensibly civilian role. In addition, they were justifiably apprehensive about installing a secretary of defense who could be perceived as partial to one service over the others.

More than a half-century later, these concerns are still highly relevant. We should ask ourselves whether the reputation of our military as a highly professional, nonpartisan institution would be tainted if its most respected leaders were allowed to seamlessly segue into political positions. That’s why, instead of simply rubber-stamping President-elect Donald Trump’s choice, it is critical that we engage in a meaningful debate before discarding this well-established precedent.

The last time a recently retired military man, the great George Marshall, was permitted to lead the Pentagon, the United States was facing the prospect of ignominious defeat in the Korean War. Even then, congressional leaders specified that his waiver was a one-time exception to the rule. While our country must confront an array of threats today, none of our national security challenges remotely compares to a massive war in the Far East. This history should inform Congress’s decision about Mattis. When it comes to something as basic as civilian control of the military, I strongly believe waivers should be granted for extraordinary circumstances — not extraordinary people.

(Jenny Starrs/The Washington Post)

Many of my fellow Democrats disagree. Recognizing Mattis’s exceptional judgment and ability, they believe he could serve as a counterweight to Trump — a partial antidote to our new commander in chief’s profound lack of expertise, experience and discipline on matters of national security. I certainly sympathize with this view. However, I am equally concerned about the kinds of decisions that will emanate from the White House if, as appears likely, the national security adviser, the homeland security secretary and the defense secretary are all former generals. The American people should demand a diversity of views and experiences — both military and civilian — in the Situation Room.

When debate on Mattis’s waiver resumes in January, a long-standing precedent will be at stake. Future generations of American leaders — perhaps facing circumstances far more perilous than our own — will look at how we dealt with this test of our commitment to civilian control of the military. Congress would be wise to uphold this time-honored principle by denying Mattis a waiver to serve as secretary of defense.