California declaring independence from the United States of America and going it alone is a colossally stupid idea.

“Yes California” soon will be gathering signatures for its advisory #Calexit folly, aiming to get it on the fall 2018 ballot. The movement is part egotistical lark, part epic hissy fit over the election of Donald Trump, and maybe — indulging the conspiracy theory du jour here, just for fun — an insidious plot to free the rest of the United States from California’s environmentalist and socially progressive influences.

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At best, it will waste millions of dollars on a losing ballot measure. At worst it will start us down a costly, intellectually draining, dead-end path into a world of overwhelming unknowns.

More important, it will waste time and influence that otherwise could be devoted to repairing the politics of the nation to which we are inevitably tied. And would be even if we were technically independent.

For the challenges of independence, let’s start with the basics: writing a new Constitution. The movement several years ago to convene a state constitutional convention to fix just a few provisions was telling. People ran screaming from the prospect, fearing what could fly out of Pandora’s box if it were opened.

We’re all California Dreamin’ for sure. The thing is, we all have different dreams.

In an oped in January, Marcus Ruiz Evans, the founder of the Calexit movement, painted a Pollyanna picture of independence. Among other things, he said, don’t worry about a military — we have the National Guard! And we’re rich!

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Yes, we have a part-time army our governor can call upon. But it is, ahem, connected to the U.S. military. (That’s why it’s “National,” see?) Of course we can start our own. It’s just a matter of cost. And of whether it will make us safer than being part of the United States. Or safe from the United States.

As to our wealth — it’s unclear whether we now send more tax dollars to Washington than we get back. It depends on what budget pockets you count. But today we are not running a State Department, negotiating treaties and trade deals, administering (and funding) our own Social Security, Medicare and on and on.

Let alone patrolling the long border on the east side of the Sierras to keep out those murderers and rapists from Nevada.

Even if California framed a coherent plan to exit, it would need a two-thirds vote in Congress and approval of three quarters of the state legislatures. Their standard will not be how happily-ever-after they can make us. It’ll be whether they’re better off without us.

If they think they are — then we’re really in trouble.

So before you sign one of those Calexit petitions, think about how much money and time you want to waste on this silliness.

Do the great state of California a favor. Just say no.