Civilian control of armed and uniformed services

Armed and uniformed services in a constitutional democracy answer to lawful civilian authority. They do not function as instruments of one political faction; they do not serve as personal enforcers of any officeholder; and they are not deployed coercively against the citizens whose taxes pay them. Military officers, federal law enforcement, and the National Guard operate under chains of command rooted in law, not in personal loyalty to any leader.

This ideal is eroded when uniformed services are treated as political assets, when federal forces are deployed domestically against US persons outside lawful bounds, when convictions for war crimes or serious misconduct are pardoned for political effect, and when officers who refuse unlawful orders are retaliated against. The principle is older than the Constitution and remains the most reliable predictor of whether a republic stays a republic: when armed forces stop following law and start following one person, the country has already changed.

Further reading: National Constitution Center Interactive Constitution — Article II (Commander in Chief clause), Third Amendment.