Peter H posted the following on the Shumshu landing here on AHF in 2005:
In general, the campaign was executed with vigour and efficiency - the Soviet forces attacking the Kurils occupied them without much of a struggle, Shumshu alone proving a costly landing. This island can be considered to be the last battlefield of World War Two. The U.S.S.R. officially declared war on Japan on 9 August and on the afternoon of the 17th, two days after Japan's surrender, Major-General Aleksei Gnechko, commander of forces on Kamchatka, launched his amphibious operations against the Kuril Islands. He was given ten days to occupy the archipelago. His flotilla carrying the 101st Infantry Division approached Shumshu and facing him were 25,000 Japanese soldiers under Major-General Fusaki, with only three aircraft to aid him.
Gnechko's armada entered the straits around 4.00am on 18 August, were spotted and assaulted into a hellish crossfire around an hour later - 13 landing craft, packed with troops sank or exploded in the first wave. Shumshu's three fighter planes did what they could in the ensuing battle - in the last kamikaze dive of World War Two one pilot crashed his plane into a Soviet destroyer escort.4 The other two also inflicted serious damage before escaping to Tokyo. By the end of 18 August, 8000 Russians held a precarious foothold on Shumshu. The Russians insist the number killed coming ashore is exaggerated, despite a firm estimate that 2000 died.
Fusaki adopted a purely defensive posture after the Russians landed and the honeycombed hills of Shumshu made it hard for their enemies to dig them out. 19 August saw the Russians on Shumshu achieve a surrender with two key Japanese batteries destroyed around 7.00am. Paramushir was taken on 25 August, Matua on the 26th, Itrup the 29th, Urup the 31st, Kunashir and Shikotan on 1 September, and the Habomai Islands on 2-4 September, all with little or no resistance. It is interesting to note that during the first days of fighting on Shumshu, the Japanese were convinced they were engaging American troops and Fusaki sent an urgent message to Imperial Headquarters in Tokyo, protesting the breach of the cessation of hostilities between the U.S. and Japan, signed on August 19.
The Shumshu landing occurred two days after Japan's surrender and was not met with the usual fight-to-the death mentality exhibited by the Japanese defending island outposts before the surrender. Indeed, it would appear that Japanese losses were under 5% of the garrison and that Soviet losses were actually heavier.
In the above list of warships handed over by the USA to the USSR in 1945, only 34 were landing craft, each designed to carry about 200 men. This would give a total lift of some 6,800 men. This, of itself, is quite insufficient to attempt a landing on a major Japanese home island with any prospect of success.
In the Shumshu operation five of these were lost. Thus carrying capacity was reduced by some 1,000 men in attacking a minor Japanese Island not defended to the bitter end.
I must therefore still question whether there really was a Soviet invasion threat to Japan proper in August 1945?