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On Thursday night, Donald Trump announced that he had selected Gen. James Mattis as his pick to become Secretary of Defense.

There are lots of arguments to be made both for and against Mattis' suitability for the role. He's had a long, lauded military career. He also has very hawkish views on Iran and extensive ties to controversial Silicon Valley company Theranos.

But the main obstacle standing in his way is simple math. U.S. law says you have to be out of the military for seven years in order to be Secretary of Defense, and Mattis only retired in 2013.

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This isn't some legal technicality. It's a longstanding rule set up by Congress as a protection to ensure civilian control over the military.

Since the Department of Defense was established in 1947, the law has required former military members to wait at least 10 years (lowered to seven in 2008) before holding the cabinet position controlling the military. Only one waiver has ever been granted, for Gen. George Marshall in 1950. The granting of a waiver to Marshall was seen as a special case, and Congress even issued a special report saying so:

It is hereby expressed as the intent of the Congress that the authority granted by this Act is not to be construed as approval by the Congress of continuing appointments of military men to the office of Secretary of Defense in the future. It is hereby expressed as the sense of the Congress that after General Marshall leaves the office of Secretary of Defense, no additional appointments of military men to that office shall be approved.

Why does it matter whether or not the secretary of defense has distance between his military service? It helps protect from a situation where the military, which is not elected, begins to set policy rather than follow it. When the military does set policy, we call that a dictatorship or a junta. That's typically not something we want in America.

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Some people in Congress agree. Sen. Kirsten Gillabrand, D-N.Y., released a statement saying that she will oppose any waiver in the Senate.

“While I deeply respect General Mattis’s service, I will oppose a waiver," Gillibrand stated. "Civilian control of our military is a fundamental principle of American democracy, and I will not vote for an exception to this rule.”

Because the waiver needs to be a full act of Congress, the Democrats actually have a shot at keeping any such waiver from reaching a 60-vote threshold in the Senate. I guess we'll find out how much Senate Democrats are "aligning" with Trump soon enough.