Soviet troop numbers versus amphibious lift for Japan

In the Second World War, did the USSR have the capacity to invade Japan?

No: the Soviets had troops, but not the Far East amphibious lift needed to mount the invasion without Allied shipping.

The USSR had the troop numbers, but amphibious shipping was the limiting resource. It lacked the Far Eastern lift required to mount the proposed invasion independently. The Allied arrangement described here has British and American forces carrying out their own landings first, then sending the invasion shipping to collect Soviet troops. Soviet participation would therefore depend on ships becoming available from another operation.