Many of Mattis’s Republican backers will make the same comparison this week, as Congress begins to consider whether or not to make an exception to the 1947 National Security Act. The law requires the secretary of defense to be “appointed from civilian life,” which now means a seven-year waiting period between retirement and appointment.

“And he don’t want them anyway,” Hayden quipped. “But he doesn’t want Joe the Plumber either, as the secretary of defense. So where can he go for unarguable expertise without buying into the pre-established political inner circle in Washington? Bing! Go to the armed forces. I think it’s not so much the love of the uniform. It’s that ‘I don’t want to go to the normal well, but I still need talent.’”

In a speech last month at the Fort Myers Officers’ Club, Michael Hayden, the former head of the NSA and CIA, suggested that Trump’s overrepresentation of the military in his appointments was simply due to a lack of a better alternative.

Why has Donald Trump picked so many generals for his cabinet? A few popular theories: Trump has patterned himself on the movie “Patton”; he spent his boyhood years at New York Military Academy; he likes the martial machismo of gold-braided uniforms; he needs to balance the ruthless image he cultivated on the “Apprentice” with a nod to sacrifice and service.

The Mattis waiver will require 60 votes in the Senate, 10 more than the simple majority needed to confirm a presidential appointment. Democrats, who hold 46 seats, could block it. But so far only one Democratic senator, Kirsten Gillibrand, has come out against the waiver, calling civilian control of the military “fundamental to the American democracy.”

Trump’s choices for defense (James Mattis), homeland security (John Kelly), and national security adviser (Michael Flynn, who does not require Senate confirmation) are all retired generals. His interior secretary (Ryan Zinke) was a Navy SEAL commander; his CIA director (Mike Pompeo) was an Army captain who graduated first in his class at West Point. Before approving a cabinet with such heavy representation from the military, senators may want to consider whether Trump is simply selecting for a value that is uniquely prized by the military: following orders.

“The supreme military virtue is obedience,” wrote Samuel P. Huntington in “The Soldier and the State.” Before confirming Trump’s military nominees, Congress should ask how far these former soldiers would go to carry out Trump’s orders and how they would disentangle the cabinet’s advisory role — as spelled out by Article II, Section 2, of the Constitution — with the more hierarchical military authority wielded by the commander-in-chief. Professor Richard Kohn, a military historian who has written extensively on civilian-military affairs, told me that under the commander-in-chief clause of the Constitution, “any disobedience is practically mutiny.” Trump has discussed the possibility of torturing prisoners and carrying out pre-emptive nuclear strikes. The Senate needs to ask Trump’s military nominees what they would do if Trump gave them such orders.

The requirement that the secretary of defense be a civilian was part of a much broader package of measures intended to institutionalize the expansion of the military in the aftermath of World War II. The U.S. military has always been under the control of civilians, or at least the civilian-elected Congress and president. The reasons behind this, and the thinking behind the 1947 rule, are laid out by a recent Congressional Research Service paper. Today, the military is so large, with six regional commands and a $600 billion budget, that if there are not clear checks on its influence, it could overwhelm the White House, the State Department, and the many other parts of the government with a role in shaping foreign policy.

Running alongside this pattern of stability is a history of Americans openly worrying about the possibility of a military coup. Among the oldest of these pessimists is the anti-federalist Brutus, who argued against the ratification of the Constitution in 1787 and 1788. For Brutus, the idea of a “large, standing army in a time of peace” was a threat to democracy, and unnecessary to deter Britain, Spain, and the native Americans along the Western frontier. He pointed out the examples of Julius Caesar and Oliver Cromwell, two generals who used their military commands to achieve autocratic power. As a solution, Brutus wanted individual state legislatures to act as a check on the use of federal military power. While this proposal went nowhere, his concerns find an echo in the Constitution itself, with its two-year limit on Congressional military appropriations, as well as the 1796 Farewell Address from George Washington, himself a retired general, with his warning: “Overgrown military establishments, under any form of government, are inauspicious to liberty, and which are to be regarded as particularly hostile to republican liberty.”

The classical view of the American military as a small force of citizen-soldiers — one which could temporarily expand during times of conflict — was forever changed by the second World War. Both sides engaged in the mass bombing of civilian targets to terrorize enemy populations and disrupt their economies. After the war, the Cold War system of interlocking alliances, with the U.S. shouldering most of the burden for the West, brought about the rise of a huge, permanent military establishment.

This period had its own Brutus: Harold Lasswell, a political scientist at Yale and a specialist in the study of propaganda. In 1941, Lasswell wrote “The Garrison State,” a dystopian paper that describes what a technologically advanced society might look if brought under total military control, “a world in which the specialists on violence are the most powerful group in society.” His nightmare state includes compulsory labor, rubber-stamp “assemblies” to “ceremonialize” the image of democracy, and a pharmaceutical industry “to deaden the critical function.” Lasswell was more prescient when he warned that “the elite of the garrison state will have a professional interest in multiplying gadgets specialized to acts of violence.”

Twenty years later, Dwight Eisenhower’s farewell speech looked warily upon “a permanent armaments industry” and, famously, “the acquisition of unwarranted influence, whether sought or unsought, by the military-industrial complex.” An early draft of Eisenhower’s speech called for “constant vigilance, and a jealous precaution against any move which would weaken the control of civil authority over the military establishment.”

The National Security Act of 1947 was, among other things, an attempt to steer away from Lasswell’s vision of the military encroaching into every aspect of civilian life, even as its budgets continued to swell through the Cold War.

In Mattis’s case, the Senate needs to take a hard look at the George Marshall analogy. Even a cursory look at the circumstances of Marshall’s appointment suggests that it does not hold up.

Marshall was Harry Truman’s third secretary of defense. Truman’s letter to Congress asking that it approve the waiver emphasized the crisis in Korea, and Truman’s belief that as a “general principle … our national defense establishment should be headed by a civilian.” As a five-star general, Marshall had organized Allied forces in Europe and the Pacific. Unlike Mattis, he had extensive post-military experience as a civil servant, having served as Truman’s envoy to China and then his secretary of state. Another difference between the Marshall and Mattis nominations is the scale of the crisis facing the U.S.; Truman’s pick came at a time when Stalin had just detonated his own atomic weapon, the Communists had just come to power in China, and a proxy war was beginning in Korea.

Eighteen months after Marshall’s appointment, in what might be the greatest civil-military crisis in U.S. history, Truman would fire his commander in Korea, General Douglas MacArthur, after he learned that MacArthur was conducting his own diplomacy and attempting to extend the war into China. The Department of Defense had only been in existence for a year—between 1947 and 1949 it was known as the National Military Establishment. On the Senate floor, Sen. Leverett Saltonstall, who voted in favor of Marshall’s waiver, said that “if solutions to the present obstacles to a more peaceful world are not obtained soon … free civilization as we know it may come to an end.” Truman, unlike Trump, was a known quantity. And he needed all the help he could get.

Charles Dunlap, a retired Air Force general, provided an alternate history of what the collapse of civilian control might look like in his 1992 essay, “The Origins of the Military Coup of 2012.” Dunlap was a lieutenant colonel and judge advocate at Central Command at the time, and the essay takes the form of a letter from an imprisoned dissident recounting the details of a Pentagon-led coup d’état. Many aspects of Dunlap’s scenario are eerily familiar—the integration of the military into counter-narcotics and anti-crime police work, humanitarian and state-building missions abroad, and a less successful Second Gulf War. Eventually, Congress demands a more centralized cross-branch military command, culminating in a Military Plenipotentiary – “General Brutus” — who carries out a bloodless coup by interrupting the constitutional line of succession and declaring himself commander-in-chief. Mattis is no General Brutus, but the Congress of 2016 would do well to consider Dunlap’s warning: “The armed forces exist to support and defend government, not to be the government.”

Mattis himself co-edited a book on relations between the civilian and military wings of society. In it, he and his co-editor write that loss of civilian control is an unlikely scenario; the larger problem is a “divergence” between a professional military and a society that does not experience or pay the costs of war. “The military could even come to consider itself a society apart, different from, and more virtuous than, the people they commit themselves to protecting, like praetorian guards at the bacchanalia.”

It has been 66 years since Congress made its only exception to the rule that the secretary of defense must be a civilian. Should Congress pass a waiver for Mattis, it would represent another bending of American norms in what has already been a season of exceptions: the Senate’s refusal to consider a sitting president’s Supreme Court nomination; a president and president-elect openly battling for control of the bully pulpit; the attempt to push through nominees who have not completed their statutory ethics reviews; a president-elect moonlighting as a media critic; a president-elect attacking the people who will be his eyes and ears. Small rules matter, especially under a president who has shown no respect for the large ones.