Quote from: Pete Nash;904859The Soviet Pacific fleet was not designed for deep water flotilla missions, but rather it was intended for littoral area denial. So on paper at the start of August they had 2 cruisers, 12 destroyers, 19 coast guards, 10 mine layers, 52 mine sweepers, 49 submarine hunters, 204 torpedo boats, 78 submarines and 1,618 aircraft (including 1,382 combat planes). In addition, the Amur river flotilla had 8 monitors, 11 gunboats, 52 armoured launches, 12 minesweepers and some other warships, which could sortie out in calmer weather.

Bare in mind that this is just the armed vessels. They had countless small freighters, fishing trawlers and other support craft which were used to carry troops, including a lot of auxiliary boats captured at the Korean ports of Yuki, Seishin and Rashin. You might think that this would be insufficient to land a force on Hokkaido, despite Soviet willingness to simply run aground 'expendable' vessels loaded with troops as a one-way assault force.  However, thanks to Project Hula, from May to July the US had very kindly supplied the Soviet navy with additional warships, including 30 Large Infantry Landing craft LCI(L), of which 5 were lost during the capture of the Kuril Islands. Atop that were 17 LCT(6)s, and 50+ LCM(3)s (although the latter I haven't been able to validate with online US Naval records).

Despite claims that the Soviets would have had troubles mounting an invasion of Hokkaido, by that time there were no fully functional Japanese naval assets left in the Sea of Japan. Everything they had left had been moved south to defend Kyushu against US invasion. The only troops left on Hokkaido were 2 divisions and a few battalions, under-equipped and supplied, having to protect an extremely long coastline. So wherever the landings occurred the Japanese would have been defeated in detail. The Soviets had learned some sharp lessons from taking the Kuril islands and they would also have unified all of their dispersed landing/support ships now that the other island chains had been taken.

Added to that, was the fact that the Soviets had absolute air superiority and the 128th Airborne Division available to support a coastal assault. So between sea landings and air drops they could have landed 2 division's worth of troops in the first few hours, after which things would only have gotten worse for the Japanese, who were cut off on Hokkaido; whilst the Soviets would have continued to shuttle yet more men to the beachhead. It was simply a matter of time.

Indeed, if not for the fact that Stalin wanted to seize the territories promised him before Japan signed the peace treaty, both Shimushu/Kuril operations would have been coordinated better. As it was, the entire campaign had been a rush job using minimal resources. Despite success, Gnechko was four days behind schedule for the planed invasion of Hokkaido, so Stalin refused him permission to continue.

If Stalin didn't have a time deadline, the only thing the US could have done to slow the conquest of Hokkaido would be to start a submarine campaign against the Soviet navy - which would have been of limited success considering the Soviets now formidable logistics chain, territorial proximity and air superiority which could have resupplied Hokkaido by plane.