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On April 23, California Secretary of State Alex Padilla approved language for a 2020 ballot proposal submitted by the Yes California Independence Campaign. The proposal will -- assuming the campaign can collect and submit signatures from 365,880 registered voters by October -- kick off a process already widely known as "Calexit" (after the United Kingdom's "Brexit" from the European Union).
That process entails three parts: Asking Californians (in 2020) if they want to "discuss" secession; if yes, asking Californians (in 2021) if they want to secede; and if again yes, asking 2/3 of both houses of Congress and 3/4 of the state legislatures to pass a constitutional amendment allowing California to leave the United States.
Whether or not that last step should be necessary is debatable, but seeing as how the last American secession resulted in a four-year war and a million dead, getting buy-in from DC and the other states might be the wisest course. Either way, if Californians want to go their own way, they should be free to do so, as should other existing states and even smaller areas and groups.
As an independent nation, California would boast the fifth largest economy in the world, and would rank 36th in population (by comparison to the world's 196 existing countries) and in the top half by area (it's larger than Hungary, Greece, or Portugal). It has its own coastline (but its secession would still leave the US with access to the west coast via Oregon and Washington). It has its own border with a country other than the US (Mexico). It relies on other states for energy and water, but making that trade international rather than merely interstate doesn't seem like an insuperable problem.
In short, California looks like an excellent test case for independence. It mostly has what it needs to function on its own.
As for relationships with other states and with a national capital 2,375 miles from its own, it's far from obvious that the people of California have so much in common with the people of Texas or Florida or New Hampshire or Wisconsin that all five states need a government in common.
Ultimately, political government itself is the problem and a system of market anarchy or panarchy (competing "public service" providers within the same geographical area) is the solution. Until we can feel our way to such an arrangement, peaceful secession, decentralization, and devolution are probably the best outcomes we can reasonably hope for.