Most people aren't taking geography into account.
Cali has an ocean on one side that is almost impossible for anyone outside the west coast to reach (and they'd probably buy Oregon and Washington's allegiance). Their other side is protected by a massive mountain range that's extremely difficult to cross. LA is surrounded by mountains and ocean, SD is between the ocean and a sea with mountains on both land sides, and you'd have to cross the freaking Sierra Nevada's to reach SF. Their populations and resources are the best protected in the country. They have massive military assets they could funnel out to attack through the north, south, or Mojave, but anyone attacking then would have to deal with a bottleneck in the mountain passes that would make their lines thinned out and easy to exploit with the high ground on either side.
Texas is the nearest real rival. Cali could attack Texas from the west via forces coming through Arizona, from the south via Mexico, from the north via Oregon, or from the Gulf via the Panama Canal. Texas would have no idea where to defend from. While Cali's only real vulnerability would be the coastline...except that Cali has more Navy bases than any other state so Texas wouldn't stand a chance there.
