The shipping gap in a Soviet invasion of Japan

Could Soviet forces have invaded Japan without Allied amphibious shipping?

Not on the required scale. They had some coastal-landing experience, but would have needed British and American lift, crews, and naval coordination for an assault on Japan.

The Soviet land offensive in Manchuria posed a different problem from crossing to Japan. A major landing in Japan is judged to have required amphibious shipping supplied by Britain and the United States. Soviet forces had experience of coastal landings in the Baltic and Black Seas, but that experience did not by itself provide the lift for a large open-water operation in the Pacific. One hypothetical arrangement has British and American ships and crews open the front before bringing Soviet troops across to join the campaign. A later exchange distinguishes short coastal crossings from operations over open water and notes that Soviet naval infantry had been depleted in land fighting. The proposed Allied arrangement is a counterfactual explanation of the shipping requirement.