George C. Marshall

A man of character, great staff officer, and leader. He put the right generals in the right arenas. And he literally outranked MacArthur which had to piss him off.

It boils down to Congressional-Intent; and since that body changes every 2 years and can be as fickle-as-a-pickle (hence the waivers) we're left with the usual political shenanigans. To understand the reason for the inspiration for the National Security Act of 1947; it's subsequent revisions and congressional waivers for SECDEF regarding recent Military Service, see the following.


This is from the National Constitution Center:
Constitution Check: Why is the Pentagon usually led by a civilian?

https://constitutioncenter.org/blog/constitution-check-why-is-the-pentagon-usually-led-by-a-civilian

Civilian control of the military is one of the oldest traditions of America’s constitutional government. It actually predates the Constitution itself; it very likely began in March 1783, when General George Washington deftly headed off a planned military takeover of the government – the episode known as “the Newburgh conspiracy.” Disgruntled members of Washington’s Continental Army were deeply upset over the government’s failure to pay the troops, and a faction was bent on marching on Philadelphia to seize the government. Washington quietly stifled the plan.

Four years later, in crafting the new Constitution, the founding generation wrote civilian control of the military into the basic document, by giving the president the role as “commander in chief” of the armed forces and giving Congress the power to declare war and to provide military funding.

Congress showed its continuing respect for that tradition when it created the new office of Secretary of Defense in 1947 when it replaced the War Department with a new Department of Defense. It stressed that the Secretary was to be chosen from “civilian life.” It explicitly provided that no military officer could hold that job until after being separated from the service for a period of years (initially ten years, later seven).

Although Congress, in waiving that restriction for the widely popular selection of George C. Marshall to lead the Pentagon in 1950, had strongly opposed any repetition of it, the lawmakers clearly do have the authority to grant a new waiver to accommodate President-elect Trump’s choice of a Marine general, out of service too recently to be otherwise eligible.

Perhaps just as historically important as George Washington’s thwarting of a military coup d’etat in 1783 was President Truman’s firing of General Douglas MacArthur in April 1951, during the Korean war. Truman’s policy was designed specifically to limit that military operation to the Korean peninsula alone, but MacArthur sought to widen the conflict to include China.

Citing the Truman limited-engagement policy, the President told the American public in a speech that “a number of events have made it evident that General MacArthur did not agree with that policy. I have therefore considered it essential to relieve General MacArthur so that there would be no doubt or confusion as to the real purpose and aim of our policy.”

Although Truman did not explicitly cite the principle of civilian control of the military, it was evident that he had no doubt of a President’s power to choose the generals to implement what their civilian superiors intended for military policy, on the ground and in the larger strategic context. After issuing his order to dismiss MacArthur, Truman later said, he went to bed and slept soundly, confident that he had done the right thing.

The decision, though, was deeply unpopular. Truman’s approval rating dropped to 23 percent, and it never recovered. Leaders of Congress scorned Truman and praised MacArthur. In fact, Congress gave him the forum of a joint session of Congress, and the general responded with lengthy remarks, seeking to justify his keen desire to have led a military campaign against China. He was interested only in victory, he said, and had been denied that chance. He ended his remarks with the still-familiar refrain, “old soldiers never die; they just fade away.”

Not once in his remarks did he acknowledge the supremacy of civilian control of military strategy, but in closing “my military career” before he had planned to do so, he implicitly acknowledged that Truman had had the last word on the subject.

This history will reemerge as the Senate, for the only the second time in history, debates whether it is prepared to again allow a President to have a general as the Pentagon’s leader.