(Bloomberg) — For decades, U.S. presidents visiting the Demilitarized Zone dividing the Korean Peninsula have followed roughly the same playbook: Peer through binoculars at North Korean border guards. Look stern. Warn Pyongyang that provocations won’t be rewarded.

President Donald Trump smashed that precedent on Sunday, meeting Kim Jong Un for the third time after a last-minute Twitter invitation that even surprised the North Korean leader. Trump then became the first sitting U.S. president to enter the isolated country, walking a couple dozen feet across the demarcation line in the DMZ.