President Donald Trump’s decision to cancel his planned summit with Kim Jong Un shifts the U.S. approach to North Korea away from a monthslong rapprochement and back to a campaign of military and economic pressure.

Mr. Trump signaled the shift on Thursday, warning of U.S. nuclear and military superiority both in a letter to Mr. Kim and in public remarks. A senior administration official later said the U.S. is seeking new ways to increase economic sanctions against North Korea.

At the same time, the U.S. must now be on watch for a resumption of provocative behavior by North Korea, which hasn’t launched a missile or conducted a nuclear test since late last year. Security experts also warn that North Korea may respond by launching cyberattacks.

The military has maintained the same U.S. force posture even as optimism rose over the planned summit, officials said, so its cancellation won’t spell force increases or heightened alerts.

“Nothing has changed,” Marine Lt. Gen. Kenneth F. McKenzie, Jr., the director of the Joint Staff, said at a Pentagon briefing Thursday. “We didn’t ramp up or down as word of this summit began to rise and, now, has ended. We’re being very steady, very straight in terms of our preparations, in terms of the readiness that we display.”