They are retired. they can fight for whoever they want.
Can, yes. Have problems doing so, yes also.
If retired, they can be recalled to U.S. active military service at any time for the rest of their life.
If they take up arms in opposition to the United States, they relinquish their citizenship.
If they serve as an officer or NCO for any country other than the U.S., they can have their citizenship removed.
I read all this and thought why publicize what
you are going to do.
Shut the F up and surprise Vlad.
Your chance of success is best by surprise.
Even a small well trained force can cripple
a large slow moving force.