> Why would there be a war?

There'd be a war because of ports, military facilities, oil, and, because, politically California may on average be more slightly liberal than the rest of the US, but overall it looks a lot like the US as a whole, with very liberal coastal enclaves and an ultraconservative inland agricultural heartland, and much of the latter would revolt from California to stay in the US in the event of an attempted Calexit.

Barring an Ecotopia-MAD-civil-war, the federal government will never give up the naval bases in California.

A number of reasons:

The legality of California seceding is dubious[1]

There is a history in the US of attempted secession leading to war

Around the world attempts are at secession are often associated with violence

There's probably more, but I don't think any of these are particularly controversial.

[1] https://en.wikipedia.org/wiki/Secession_in_the_United_States...

Even if there wouldn't be a physical war, there would be no reason to expect free trade, free movement, shared currency or a stable replacement currency.