The Military should investigate and prosecute any servicemember or officer supporting any secessionist cause. They swore an oath and if they choose to violate it, they have no place in the Military nor in Society. America has no time for oath breaking cowards.
They are guilty of a felony, violation of Article 94 of the Uniform Code of Military Justice.
The article calls for the death sentence, but it should be commuted to a felony conviction, immediate forfeiture of pay and allowances, and confinement until discharge. Being convicted of a felony is worse than death.