Donald Trump is scheduled to meet North Korean leader Kim Jong Un Tuesday in Singapore after a year of turbulent foreign-policy maneuvers built on the U.S. president’s willingness to take outsize risks.

He has riled European leaders by quitting the international nuclear deal with Iran, is threatening tariffs and other punitive trade measures against allies and is contemplating a summit with a perennial U.S. adversary, Russian President Vladimir Putin. After calling Friday for Moscow’s readmission to the Group of Eight leading...