I read this somewhere: CONGRESS SHALL MAKE NO LAW....
I guess that's just a dead letter, eh?
That's not entirely accurate, LB. While on active duty, you can voluteer to work in a political campaign, you can speak, and you can, of course, vote. However, you must do so on your own time, you must do it in civilian clothes, and you must give no indication whatsoever that you are representing the military.
It is always against regulation for a member of the military to speak contemptuously of the Commander in Chief.
Like Marlowe says, though, they'll never stop troops from complaining about the brass. It's a given. Therefore, speaking contemptuously, in my humble opinion, does not include griping about perceived lousy decisions.