The dropping of atomic bombs on Japan.
While island-hopping was restoring strategic territories to the Allies' side (reclaiming those places from Japanese control), and while the bloody battles on the Japanese home islands of Iwo Jima and Okinawa ultimately were Allied victories, Japan was not ready to surrender,. The USA considered a land invasion of the main island of Japan too high a cost to pay, and chose to use the atomic bombs to force a final end to the war in the Pacific. Those bombs brought about Japan's surrender.
