The Soviet Union invaded the Japanese portion of Sakhalin Island, also known as Karafuto Prefecture. The invasion was part of the Soviet–Japanese War, a minor campaign in the Asian theatre during World War II.
Background Following the Japanese invasion of Sakhalin in 1905, control of the island was split according to the Treaty of Portsmouth, with the Russian Empire controlling the northern half and the Empire of Japan controlling the portion south of the 50th parallel north. It was known in Japan as Karafuto Prefecture and the Northern District.
During the Yalta Conference in 1945, Soviet Premier Joseph Stalin pledged to enter the fight against the Empire of Japan "two or three months after Germany has surrendered and the war in Europe is terminated." That would create another strategic front against Japan, which was deemed necessary to end the war. As a result of their participation, the Soviets would be awarded South Sakhalin and the Kuril Islands, among other concessions. The United States would aid the Red Army in Project Hula in preparation for the invasion.
On 5 April, the Soviets formally repudiated the Soviet–Japanese Neutrality Pact.
On 9 August, the Soviets launched a full-scale invasion of Manchuria, which started the Soviet–Japanese War. That war began three days after the United States atomic bombing of Hiroshima, and it included plans to invade South Sakhalin. The main purpose of the invasion was to clear Japanese resistance and to be prepared within 10 to 14 days to invade Hokkaido, the northernmost of Japan's home islands.
Order of battle
Soviet Union 2nd Far Eastern Front (commanded by General of the Army Maksim Purkayev) 16th Army (commanded by General Leonty Cheremisov) 56th Rifle Corps 79th Rifle Division 2nd Rifle Brigade 5th Rifle Brigade 214th Tank Brigade 113th Rifle Brigade 255th Composite Aviation Division (106 aircraft) Pacific Fleet (commanded by Admiral Ivan Yumashev) Northern Pacific Flotilla (commanded by Vice-Admiral Vladimir Andreyev) Pacific Fleet naval aircraft (80 aircraft) 365th Naval Infantry Battalion
Imperial Japan Army: 88th Infantry Division of the Japanese Fifth Area Army (commanded by Lieutenant General Kiichiro Higuchi) Karafuto fortified area of the Border Guard. (17 bunkers, 28 artillery and 18 mortar positions, and other facilities; garrison of 5,400 troops.) Detachments of Reservists
Navy: Naval Infantry Northeast Air Fleet Karafuto Region ground defence units some units from Ominato Defence Force久堀（1992年）、44頁。 part of the Soya Defence Force – the icebreaker Odomari, the special gunboat Chitose Maru, the Soya Defence Guard Station, and the Nishinotoro Defence Guard Station. Toyohara Navy Office – Military Attaché: Rear Admiral Koichi Kuroki
Invasion
General situation Before the outbreak of the Japan-Soviet war, the Japanese military's deployment was divided into the northern area (Shikika and Esutoru districts) and the southern area (Toyohara and Maoka districts). The 125th Infantry Regiment was stationed in the northern area, and the main force of the 88th Division was in the southern area. Both were tasked with conducting defensive operations in a prolonged battle, either against the Soviet Union or the United States. The northern area was covered by tundra, and the transportation network was underdeveloped. From the border to the vicinity of Kamishikika Station, there was essentially one military road and railway, making it easy to predict the enemy's course. The 88th Division, on-site, had been requesting a reorganization to focus more on the Soviet threat since late June. Finally, on August 3, they received permission to intercept in case of a Soviet invasion.中山（2001年）、63頁。
On August 9, the Soviet Union declared war on Japan, but the order for the Soviet 16th Army to invade Sakhalin came on the night of August 10. The operation plan was divided into three phases: In the first phase, the 1st brigade (primarily the 79th Rifle Division and the 214th Tank Brigade) would break through the border security line. In the second phase, they would capture Kotono Station, which was considered a "fortress",古屯には歩兵第125連隊第2大隊の兵舎があり、北側の幌見峠には北樺太侵攻作戦を援護するための鉄筋コンクリート製構築物などから成る陣地が建設されていた。ただし、実戦では日本軍は幌見峠にほとんど守備兵を配置しなかった中山（2001年）、59、123頁。 and in the third phase, the 2nd brigade (primarily the 2nd Rifle Brigade) would rapidly advance to occupy South Sakhalin. Two brigades from the border area were the main axis, and supplementary amphibious operations were planned at Tōro and Maoka (Holmsk). The delay in the Soviet attack after the war began allowed the Japanese side some flexibility to reorganize forces for the Soviet threat. However, the Soviet forces faced intense resistance during the first phase, and delays in gathering forces for the second phase, the Kotono capture, ensued.
The Japanese 5th Area Army received the news of the Soviet participation early on August 9 but instructed its subordinate units to refrain from active combat actions. This restraint order was lifted the next day, but due to communication delays, the frontline units did not receive the update in time, leading to overly passive tactical actions on the front lines.中山（2001年）、83–84頁。 After the restraint order was lifted, the 5th Area Army instructed the 1st Air Division's 54th Fighter Squadron to advance to Ochiai Airfield, but bad weather prevented the operation. Meanwhile, Soviet aircraft also struggled with the weather but managed to provide ground support. On August 13, the 5th Area Army decided to send reinforcements—three battalions from the 7th Division in Hokkaido—and even planned a counter-invasion of North Sakhalin (scheduled for August 16). However, on August 15, following the acceptance of the Potsdam Declaration, an order was issued from the Imperial General Headquarters to cease active offensives (Order 1382).中山（2001年）、112–113頁。
The 88th Division on-site initiated a defense mobilization on August 9 and began recruiting special defense forces. On August 10, a combat command post was established at Kamishikika, with several staff members sent. By August 13, the National Volunteer Combat Teams were also summoned. The recruitment of volunteer combat teams from among the general populace was unique to the Sakhalin campaign, with the aim of giving the appearance of military readiness to deter the Soviet advance.中山（2001年）、111頁。 The division learned of Japan’s acceptance of the Potsdam Declaration on August 15 via the imperial broadcast.
At 4 pm on August 16, the Imperial General Headquarters issued Order 1382, directing all military forces, including the 5th Area Army in Hokkaido, to cease combat immediately, except for defensive actions in case of an enemy attack before ceasefire negotiations were established. The deadline for this order was within 48 hours. However, on August 16, when the Tōro landing operation began, the 5th Area Army’s Commander, Major General Kiiichiro Higuchi, ordered the 88th Division in South Sakhalin to continue fighting and block the Soviet advance, issuing a command to defend South Sakhalin at all costs. This order was based on the fear that the Soviet forces might concentrate at Ōtomari in South Sakhalin and advance as far as Hokkaido. However, according to the War History series, it is unclear what instructions were given by the 5th Area Army as there is no original documentation on the matter. The War History series does support the idea that Division Commander Mineki’s recollections align more closely with the stance of the 5th Area Army, suggesting the order to defend South Sakhalin might not have been explicit.
When Major Yoshio Suzuki, the Chief of Staff of the 88th Division, received Higuchi's combat order on the afternoon of August 16, preparations for ceasefire were already underway, such as the disbandment of mobilized units, the discharge of some troops, and the disposal of the regimental flag. The division had no tanks or aircraft, and certainly no anti-tank or anti-aircraft weapons that could withstand Soviet forces. In light of this, Suzuki quickly ordered the re-arming of units and the occupation of positions. Civilians, at this point, were already voicing complaints, wondering if the military was still going to continue fighting.
On the west coast of northern South Sakhalin, in Esutoru (Esetoru), Soviet soldiers, thinking that there were no Japanese defenders, landed without much aggression. However, they were met with gunfire from Japanese troops, resulting in the deaths of about seven Soviet soldiers. The Soviets retaliated, marking the first combat in Sakhalin after the war ended. On August 14, a large-scale blackout occurred in Esutoru, and the local Japanese forces were unaware of the emperor's broadcast on August 15, with the higher-ups in the division attempting to hide the fact of the war's end.
From the Soviet perspective, the Emperor's announcement of Japan's acceptance of the Potsdam Declaration was viewed as a general surrender. They maintained that Japan was still resisting and that it would only be considered a true surrender when the Emperor commanded the military to cease combat and lay down their arms. Despite this, both sides were willing to engage in ceasefire negotiations. Japan demanded that Soviet forces halt their occupation at the current positions, warning that further advances would be met with resistance. The Soviet Union, adhering to its demand for unconditional surrender, was determined to push forward and continue occupying the territory. The Japanese forces in South Sakhalin did not comply with disarmament or surrender and continued to resist. Even after August 16, Soviet forces continued their military operations to advance. U.S. General Douglas MacArthur intervened, urging the Soviet General Staff to cease attacks (temporarily halt their advance), but the Soviets responded that whether they would cease operations depended on the local commanders, as the Japanese forces had not complied with the advance.
The Soviets had originally planned to invade South Sakhalin and the Kuril Islands after evaluating the progress of the battles in Manchuria. However, since the war situation in Manchuria developed more favorably than expected, the Soviet High Command decided on August 10 to begin a full-scale invasion of Sakhalin. Cherevko (2003) argues that the continuation of combat by Japanese forces in both Manchuria and Sakhalin led the Soviet forces to intensify their attacks.Cherevko (2003), 77–78頁. On the other hand, according to Nakayama (2001), the reason the Soviet Union persisted with the invasion of southern Sakhalin was to prevent Japan's retreat from Sakhalin and secure bases for the eventual occupation of northern Hokkaido. On August 18–19, Soviet Far Eastern Commander Marshal Aleksandr Vasilevsky ordered the occupation of Sakhalin and the Kuril Islands by August 25 and the northernmost part of Hokkaido by September 1.スラヴィンスキー（1993年）、70–72頁。 In the border area near Gutun, Soviet troops launched a general attack on 16 August, but were unable to overrun the main positions until the 125th Infantry Regiment of the Japanese garrison was disarmed on 19 August following an immediate ceasefire order. Soviet forces also made a tower road landing on 16 August, but the landing force's advance was sluggish. Traffic routes were congested with refugees, and Japanese forces often abandoned attempts to stop enemy forces by destroying bridges and other means. During this period, the Japanese negotiated ceasefires in various locations with halts at their current positions, and Division Commander Mineki himself went to the northern sector to negotiate, but all halts to the advance were refused by the Soviets, often resulting in military attachés being shot dead.
Karafuto line