Ok, state X secedes. Who fights them?
State X secedes and they fight amongst themselves for power. For example, if California seceded, there would in all likelihood be a civil war within California; first the lefties would attack those conservatives who did not submit or did not leave the state; then the purges where lefty factions would fight each other over ideological purity and ultimate control. I was born and raised in California, and it used to be paradise. I left back in 1979. However, I still know people in California who would “not go gently in to that good night” in the event the left turned on them.