Honshu was the Japanese home island targeted by Operation Coronet, the second phase of Operation Downfall.
Coronet was planned as an invasion of the Kanto Plain near Tokyo on Honshu, Japan's largest and most populous main island. The operation was tentatively scheduled for spring 1946, after the planned invasion of southern Kyushu in Operation Olympic.
The Kanto Plain was considered a crucial objective because it contained Tokyo and other major political, industrial, and transportation centers. Allied planners expected Coronet to be larger than Olympic and to require a massive concentration of ground forces and naval support.
Coronet was never launched. Japan accepted the Potsdam Declaration in August 1945 and formally surrendered on September 2, making the planned invasion unnecessary. The island distinction matters: Olympic targeted Kyushu, Coronet targeted Honshu, and Downfall was the name for the complete two-stage invasion plan.