This concurred with Madison in Federalist #43. He had argued that the Constitution was a compact (contract). Under contract law, a party may remove himself from a contract only if all other parties to the contract concur. The one exception is if an action is taken that voids the contract.
Madison made this argument when we had only 13 states. He argued that if even one state wished to leave, the Union would have to be dissolved to accomplish that, and it would require the unanimous consent of the other 12 parties.
Lincoln's position was more moderate. Because we had so many more states than in Madison's day, Lincoln argued that a constitutional amendment would be the proper tool for dissolving the Union and then re-forming it, minus the states that chose to leave. This would require the concurrence of three fourths of the states, not unanimous consent. Arguing that unilateral secession was unconstitutional and thus forbidden, he asked the southern states to rescind their declarations of secession, send their senators and representatives back to Congress, and negotiate their way out of the Union via an amicable divorce.
After Lincoln's assassination, passions in Congress got out of control. Ten southern states were rolled into five military districts and effectively expelled from the Union. This was a violation of every principle for which Lincoln had prosecuted the war.
The states were told that they could restore their statehood only by ratifying the 14th Amendment. There were several constitutional problems with this. Only states can ratify constitutional amendments, not territories, not military districts -- which do not exist within the Constitution -- not foreign countries, and not the UN. As military districts they could not ratify, but in the heat of the moment, Congress made this a condition of their re-entry.
In 1939, these issues could have been peripherally addressed by the Supreme Court in Coleman v. Miller, but the Court chose to keep this can of worms welded firmly shut. The Civil War had ended only 74 years earlier, and the wounds were still fresh. No one wanted to seriously challenge the validity of the 14th Amendment.
The military district option has never been litigated, which means that it is still technically an option, even if it would be unwise to invoke it.
The proper way for California to leave the Union would be for a constitutional amendment to be proposed by Congress (or a Convention of the States) and ratified by 38 states, either by state legislatures or by state ratifying conventions as chosen by Congress under the provisions of Article V of the Constitution. Disunion negotiations in Congress would include disposition of land acquired by the federal government from the state for military bases and payment of the state's share of the national debt.
There is another factor. If the Union is divisible, then California is divisible. The San Francisco Bay Area and the City of Southern California wish to be independent, but the rest of the state is not so inclined.
There is no “proper” way. If the people of California vote to secede then they get to go their own way. No Constitutional Amendment needed as the US Constitution is silent on the issue. Any other shenanigans by the other 49 states will not be tolerated, nor should it be.
Thank you for your grasp of the Constitutional issues involved.
There are so many considerations for anything this catastrophic.
Would Californians lose their ability to freely move within the other States of the Union? What about property owners who don’t want secession - they own the land. And commerce between States. Who would own the highways? My mind is in chaos!!!!!!!!!!!!!!!!!
I agree, California cannot secede no matter how hard they stomp their feet. But what California libs are forcing, is a revival of the push to split CA into 7 proposed states. Leaving LA and SF having to negotiate with other states for things such as water rights. Which would warm the cockles of my heart.