What was Japan’s planned all-out defense of Kyūshū against Operation Olympic called?

Japan’s planned all-out defense of Kyūshū against Operation Olympic was called Operation Ketsugō.

Japanese military leaders developed Ketsugō in anticipation of an Allied landing on the home islands. Rather than relying mainly on defense in depth, the plan concentrated forces and suicide weapons near likely landing beaches in an attempt to destroy the invasion fleet and defeat the first assault.

The strategy envisioned thousands of aircraft, including kamikaze planes, attacking Allied transports before troops and supplies could come ashore. Japan also prepared suicide motorboats, naval forces, and civilian defense formations for the expected battle.

Ketsugō was based partly on the assumption that the Soviet Union would remain neutral. That assumption collapsed when the Soviet Union declared war on Japan on August 8, 1945. Japan surrendered before Operation Olympic began, so Ketsugō was never tested in combat.