[Civilian control of the military](https://en.m.wikipedia.org/wiki/Civil_control_of_the_military#:~:text=The%20concept%20of%20civil%20control,%2C%20societal%20values%20and%20norms%22.) is an important guardrail against military coups.
In the US, the Secretary of Defense may not have served in the military in the seven years leading up to their nomination (by The Executive Branch).
This may be waived by the congress (the Legislative Branch) but it is unusual for someone to come directly out of military service and run the military.
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