They certainly can't leave the Army if the Government starts an unjust war... Hell, people gotta sue to get out, half the time. During Vietnam, people didn't even have a choice to join up or not.

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"It is the Right of the People to alter or abolish the Government"
Declaration of Independence
"Never trouble another for what you can do for yourself."
Thomas Jefferson
"If you put the federal government in charge of the Sahara Desert, in 5 years there'd be a shortage of sand."
Milton Friedman