You’re assuming that if they break apart, they wouldn’t trade with each other anymore. That’s not what happened with the USSR, and it’s not what happened with Brexit, and it’s not what would happen if the US split apart.
The federal government won’t be able to do jack shit if California, Oregon(maybe not), and Washington decide to stop giving it money. Sending in the military wouldn’t work, there’s far too many conflicts of interest within the troops. People who were born there, or have family there, or friends.