After secession (and maybe during the process of seceding) states would naturally enter into international pacts with each other, with the remaining union and with other nations. Military, trade, travel and other agreements might be very similar to or very different from current state/federal and state/state relations. It could work out well for everyone.
"When in the course of human events it becomes necessary for one
people to dissolve the political bands which have connected them with
another ..."
So begins the Declaration of Independence of the 13 colonies from the king and country to which they had given allegiance since the settlers first came to Jamestown and Plymouth Rock.
The declaration was signed by 56 angry old white guys who had had enough of what the Cousins were doing to them. In seceding from the mother country, these patriots put their lives, fortunes and honor on the line.
Four score...
Take Texas. Energy rich, accessible ports, existing pipelines, refining capability... States that embrace envirofascism can purchase energy from Texas, boosting Texas' revenue while supporting their less-industry-friendly utopias.
The inverse, those less industrial entities may focus on finance, or technology, or... whatever.
It's a diversified, specialized world economy. Why not embrace that?