The story of modern California really starts with the Treaty of Denver. It left California isolated from the rest of the country, both physically and politically. An island of the former United States with the ocean on one side, and hostile enemies on all the others. Mismanagement of VITAS 2 and the tech crash only fed into the brewing resentment.
The Act of Union merged Canada and United States into one nation - but stirred further bad feelings among those areas culturally or geographically isolated. California was left further cut off as the country grew. It began when the southern states left in protest of the newly formed UCAS to create the Confederated American States. Then a year latter, Tir Taingire seceded from Salish Council. Secession Fever, they called it.
California's politicos brought up the idea of succession for the next two years after the CAS was born. Multiple resolutions were brought forward by the state legislature requesting succession, but were squashed by the then-Governor Treacle each time the proposal was brought forwad. But the matter was forced when UCAS President McAlister decided to retaliate to the political saber-rattling by having California removed from the UCAS. Simultaneously withdrawing federal forces and benefits from the state, with the idea being it would teach the rebellious state a lesson.
(It should be noted for posterity that McAlister is the same gentleman who didn't believe in magic when the evidence stared him in the face – right up to the Great Ghost Dance bit him in the ass.)
In response to the UCAS withdrawal, Aztlan quickly struck hard from the south to claim new territory in the form of San Diego. The newly formed Tir Taingire nation attacked from the North to push into Redding and the crescent. Those forces that had remained behind were inadequate to fight the war on two fronts. In desperation, the newly-minted President Treacle of the new nation reached out to Japan – a long time friend, with considerable corps and property. A bluff, to provoke the CAS or UCAS into stepping in to aid them rather then let Japan get a foothold.
Japan's aid came before anyone could react. In the form of imperial military – but not attacking either invading force, but instead occupying San Francisco under Japanese Imperial Martial Law. The Emperor and Diat would take no risks on their corp property being lost to either Aztlan or Tir (both hostile toward Japan). Aztlan halted their push voluntarily after taking San Diego – Tir Taingire was slowed by guerrilla warfare by locals and gangers.
Modern California has reverted to the ways of the old 19th-century frontier. Most remote communities are a law unto themselves, responsible for their own defense and administration... and take orders from no one. The Cali Free State Congress has little authority in many areas, outside of the remaining non-occupied urban centers and the Central Valley.
This can be good and bad for a Shadowrunner on the move. Good, in that there's little national enforcement of laws in many areas and making it easy to fall between the cracks. Bad in that many areas have free reign to bury and forget any outsiders that they don't like the looks of. Which can be human, meta or magic user depending on who the locals are. California is a patchwork of tiny enclaves and communities – many of them hostile to one another.
There's three ways in to the country, all of them dangerous.
Travel by vehicle or overland is somewhere between “risky” and “batshit insane” depending on your standards for getting shot in route. All ground travel into the country has to go through one enemy country or another. Chose your poison – NAN, Tir or Aztlan.
Ocean travel is generally preferred for cargo, rather then people. This is for many reasons – the chief being that the ocean is a damned scary-ass place after the Awakening.
Air travel is preferred for most people transit, but that posses another issue. High security dominates three of the four major airports, with Oakland airport remaining the popular port of call for Shadowrunners with sketchy data. Especially with the Japanese occupation of San Francisco, it remains the air port for people wishing to pass under the radar. The LAX airport no longer exists, due to extensive pollution of the old airport and fighting around the Inferno.