SAN FRANCISCO — The sale of recreational cannabis became legal in California on New Year’s Day. Just four days later, the Trump administration acted in effect to undermine that state law by allowing federal prosecutors to be more aggressive in prosecuting marijuana cases.

A memo by Attorney General Jeff Sessions on Thursday was widely interpreted in the nation’s most populous state as the latest example of Trump vs. California, a multifront battle of issues ranging from immigration to taxes to the environment.

And on marijuana, once again California reacted with defiance.

“There is no question California will ultimately prevail,” Gavin Newsom, the lieutenant governor of California, said. “The public has accepted legalization’s inevitability. It will be very difficult for Sessions to bring us back to a mind-set that existed five years or a decade ago.”

The head of California’s Bureau of Cannabis Control, Lori Ajax, said legalization would proceed as planned, “consistent with the will of California’s voters.”