Montana Police arresting ATF agents sounds at least about as likely your US Govt invasion of Montana. And how exactly does the “Feds overrun Montana pretty easy” part work? You know you can’t even begin to predict how this would play in the other states.
It’s a fair bet some other states would be outraged into supporting behaviors. Including the demands their own senators and congressmen would face from their outraged constituents. Think the tea party was a turn out? Wait and see what Tanks rolling through Montana would do. Talk about mass anger! Feds wont dare make a “Concord” move.
Its definately not as easy as “line up the panzers and roll across montana”.
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.