As for over running Montana, if the governor decides to secede, it wouldn't take much for the Federal army to roll over it. Remember what happened in Iraq to the guys who had more than just small arms.
Now, that would probably be a watershed moment, and the fallout of it would be very interesting, as I suspect many states would leave and we could have a civil war. But if Montana goes it alone (or any state) the Federal army will win.
Let's see a state pass a law requiring that any and all federal authorities who wish to carry a firearm within the boundary of said state, must be in possession of a "right to carry" permit issued and authorized by that state's governor.Other states are preparing similar legislation, but I would suggest one of them take the Tenth Amendment challenge a major step forward.