The 'Hokkaido Offensive Operation' was the Soviet unrealised undertaking to seize the northern part of Hokkaido, the most northerly of the Japanese home islands (24 August 1945).

US opposition and doubts within the Soviet high command led to the plan’s cancellation before the invasion was scheduled to begin on 24 August.

In the closing stages of World War II, the USSR declared war on Japan in accord with Iosif Stalin’s secret agreement with the Western allies at the secretly agreed at the 'Eureka' and 'Argonaut' 'big three' conferences at Tehran and Yalta respectively. The Soviet declaration of war was a major factor in Japan’s decision of 15 August to surrender. Although the Western allies ceased all hostilities against Japan upon the surrender, Stalin ordered his forces to continue fighting in order to seize more Japanese territory and thereby place the USSR in a stronger bargaining position for the occupation of Japan.

In the 'Sakhalin Offensive Operation', Soviet forces took the southern half of Sakhalin island, whose sovereignty had been been contested between Japan and Russia, later the USSR, for a century and had been the scene of fighting during the Russo-Japanese War of 1905. By 1945, the island had been divided between the two countries, the Japanese southern half being the Karafuto Prefecture. The Soviets then captured the Kurile islands group in the 'Kurile Islands Offensive Operation' starting on 18 August, three days after the Japanese surrender.

While they planned the seizures of Sakhalin and the Kurile islands group, lying to the north-west and north-east of Hokkaido island respectively, the Soviets considered it necessary to control Hokkaido, or at least those parts of the island bordering the Sea of Okhotsk, in order to secure their newly seized territories. The Potsdam Declaration of the previous month, however, had set out that post-war Japan would control its four main home islands of Hokkaido, Honshu, Kyushu and Shikoku, so any Soviet annexation or even occupation of Hokkaido would have likely provoked heated opposition from the other Allies.

Marshal Sovetskogo Soyuza Aleksandr M. Vasilevsky, the commander-in-chief of the Soviet Far Eastern Command, envisaged the capture of the northern two-fifths of Hokkaido, an island with an area of 32,210 sq miles (83424 km²), by landing at the small and remote port of Rumoi on the island’s western coast and occupying everything north of a line extending to the south-east from Rumoi to Kushiro on the island’s eastern coast. For this undertaking the army was to commit, in the first stage, General Major Leonty G. Cheremisov’s 16^th Army in the form of the 342^nd and 345^th Divisions of General Major Fyedor F. Borisov LXXXVII Corps. The invasion was to be launched from Sakhalin, which was also to host Glavny Marshal Aleksandr A. Novikov’s air forces and the Pacific Fleet ships which would support the land forces. Even with US ships provided to the USSR in 'Hula', Admiral Ivan S. Yumashev’s Pacific Fleet did not have sufficient transport capability to carry both divisions from Sakhalin island in one lift, so the assault force was therefore to be delivered in two lifts. Yumashev planned to start the Rumoi landing at 05.00 on 24 August.

The Soviet high command dictated that although logistical preparations should go forward, the invasion should not begin without explicit authorisation from the highest-level headquarters.

The Japanese forces on Hokkaido were the army’s 5^th Area Army under the command of Lieutenant General Kiichiro Higuchi with Lieutenant General Saburo Hagi as his chief-of-staff: the 5^th Area Army comprised the 7^th Division, 42^nd Division, 101^st Independent Mixed Brigade, 22^nd Tank Regiment, 1^st Air Division and 20^th Air Brigade. Also on the island were elements of the navy’s 12^th Air Fleet and the militia forces of the Volunteer Fighting Corps.

President Harry S Truman of the USA was prepared to accept the Soviet annexation of Sakhalin and the Kurile islands group, which therefore remained part of the USSR after the war, but strongly opposed any Soviet presence on Hokkaido. The Potsdam Declaration intended that the entirety of the home islands be surrendered to the Western allies' forces commanded by General Douglas MacArthur, with none passing into Soviet hands, so Truman refused to allow the Soviets to participate in the occupation of Japan. Furthermore, concerns were raised within the Soviet high command that an invasion of Hokkaido would be impractical and unlikely to succeed, as well as violating the Yalta Agreement.

The invasion was therefore cancelled on 22 August, two days before it was scheduled to start, and the Soviet forces instead concentrated on the seizure of the Kurile islands group.

It is generally agreed that an invasion of Hokkaido would not have succeeded. Factors include the small number of Soviet transport ships, the very limited number of Soviet ground forces planned for the invasion, and the availability of Japanese air power, including kamikaze aircraft, to contest a Soviet landing. Soviet forces suffered heavy losses in the 'Battle of Shumshu' during the seizure of the Kurile islands group, and this suggests that similar problems would have beset any invasion of Hokkaido.

It is also generally agreed that the Japanese forces on Hokkaido would have resisted fiercely any attack after their country had surrendered, and that the small, hastily-assembled Soviet forces would have been unable to hold out against them. Because the Soviets thought the Japanese would not contest a landing after they had already surrendered, they assembled a relatively small force of two divisions, much smaller than the four field armies, totalling about 12 divisions, which Marshal Sovetskogo Soyuza Georgi K. Zhukov, the deputy commander-in-chief of the Soviet armed forces, estimated would be necessary for a full-scale conquest. However, after the Japanese fiercely defended Shumshu three days after the surrender, the Soviets were forced to reconsider this assumption.

At least one historian believes, however, that despite serious Soviet deficiencies in shipping capacity and air cover, the Soviets could have succeeded because the Japanese defences were concentrated in the south to face the Americans rather than in the north to face the Soviets.