Okay, US can play ball too. First, restrict all US government from doing business in California—no conferences, no new business. Maybe begin moving out government operations. End federal funding of sanctuary cities, counties, and states, such as California. Bet they change their tune really quick. California would die without the federal government and its tax money, its operations.
The US may play table tennis or a friendly game of golf, but it doesn't do hardball. If this gets serious we will not only let them go, we will pay all the bills, like enabling parents acquiescing to the demands of a teenager who requires independence but needs help with the rent, the car payments, the insurance, the tuition, the textbooks, the taxes, the interior decor, and of course the abortionist. Uncle Sam will cover all that because Auntie Sam will insist on it. There's some sort of "human right" to be spared consequences.
There are over 100 military installations in CA. The state has great strategic importance to the nation. These CalExit people are certifiable.