How this would go down:
I'm going to start with California because it's the obvious power player here with a significantly higher GDP than all other states. (2.28 Trillion) Only Texas comes somewhat close (1.60 Trillion) and New York follows in third place (1.35 Trillion).
First of all don't get your hopes up, California would never be divided. * Many businesses have the bulk of their assets in California however those assets are somewhat distributed throughout the state. * The tech, media, energy, agriculture, and trade industries all benefit from tight integration that already exists. * Internal Animosity isn't that large in California, People might love to associate with NorCal or SoCal but when it gets down to it Californians are more connected with the state as a whole than they are with any given region. If you've heard about that bill that someone introduced into the state legislature to split the state up don't take that seriously. The vast majority of Californians thought the idea absurd and the guy who devised the bill is largely considered anywhere from crazy to just a bit stupid.
As others have said California would go north first to get water. There would be three stages of expansion.
California Annexes Washington and Oregon and Hawaii. There probably wouldn't be any fight, state governments would realize that they have no chance of winning a war and economically they would benefit greatly from becoming part of California. They also are culturally very similar and California has the cultural advantage because it has Hollywood and significant dominion over the internet. Hawaii is also completely dependent on California for it's food. Also there are a ton of wealthy Californians who have significant interest in Hawaii. (Think Larry Ellison CEO of Oracle) California would have sovereignty over the Pacific with the Pacific Fleet and could blockade any trade across the pacific. This would keep Canada from interfering as it depends on it's ports in B.C. and would ensure that any nations on the wast coast of the entire americas don't mess with California if they want to have access to any shipping.
California Annexes Alaska, Arizona, Idaho and Nevada. Nevada's economy is dependent mostly on California and it can't resist because it could have it's tourism easily stopped by California and it's subordinate states. Idaho would submit instantly after seeing all the states around them get annexed, and if not the state could be secured militarily with insignificant effort or cost on California's part. Arizona has natural resources and various assets that California would want but it's biggest use would be to secure some territory in-between California and Texas.