US invasions of Japan seem to be generally considered difficult, very bloody, and even risky: even with the US Navy in support, air support including waves of B29s, chemical/germ warfare, US V1-type rockets fired in their thousands, and atomic bombs used in tactical support.
However, when the question of a Soviet invasion comes up, it seems assumed that the USSR could invade, occupy and set up a communist regime in the North of the country. Surely the USSR had a much weaker navy, among other things?