There is no principle of civilian leadership of the Military

There is no principle of civilian rule over the military in the Constitution. It may be a political tradition, but nowhere is it stated so in the Constitution. What it does say is that the President is Commander in Chief, the Congress has the authority to raise, equip, staff, and provide for the regulation of the military, and the Senate has the authority to approve the appointment of officers.

The states have the right to maintain a well regulated militia but no standing troops or ships of war in peace time, which forces may be called to Federal service. That the President is Commander in Chief seems to me that the President has a military rank, and is not a civilian insofar as the military is concerned........

There is no provision anywhere else, save by law passed by Congress, that requires or places a civilian anywhere in the chain of command. Given the disastrous terms of such Secretaries of Defense as Louis Johnson, McNamara, and Rumsfeld, one could build a real good argument to place qualified military personnel in those positions.”