The Soviets and Japanese, including their respective client states of Mongolia and Manchukuo, fought in a series of escalating small border skirmishes and punitive expeditions from 1935 until Soviet-Mongolian victory over the Japanese in the 1939 Battles of Khalkhin Gol, which resolved the dispute and returned the borders to status quo ante bellum.

In the aftermath of the First Sino-Japanese War and the Eight Power Intervention against the Boxer Rebellion in China, the Qing hold over Manchuria and Korea had weakened significantly, leading to both the Russian and Japanese Empires vying for control over the territories.

The Russo-Japanese War of 1904 to 1905 began when the Empire of Japan (led by Emperor Meiji) launched a surprise attack on the Russian Pacific Fleet stationed at Port Arthur on the Liadong Peninsula.

Following Hirohito's Japanese invasion of Manchuria in 1931–1932, (after Taisho's death in 1926) violations of the borders between Manchukuo, the Mongolian People's Republic and the Soviet Union took place frequently.

Between 1932 and 1934, according to the Imperial Japanese Army, 152 border disputes occurred, largely because the Soviets infiltrated Manchuria for intelligence purposes.

To make matters worse, Soviet-Japanese diplomacy and trust had declined even further, with the Japanese being openly called "fascist enemies" at the Seventh Comintern Congress in July 1935.

[4] On 8 January 1935, the first armed clash, the Halhamiao incident (哈爾哈廟事件, Haruhabyō jiken), occurred on the border between Mongolia and Manchukuo.

In October 1935, nine Japanese and 32 Manchukuoan border guards were engaged in setting up a post, about 20 kilometers north of Suifenho, when they were attacked by a force of 50 Soviet soldiers.

[7] On 19 December 1935, a Manchukuoan army unit engaging in a reconnoitering project southwest of Buir Lake clashed with a Mongolian party, reportedly capturing 10 soldiers.

More small attempts to dislodge the Manchukuoans from their outposts occurred in January, with the Mongolians this time utilizing airplanes for recon duty.

[8] In February 1936, Lieutenant-Colonel Sugimoto Yasuo was ordered to form a detachment from the 14th Cavalry Regiment and, in the words of Lieutenant-General Kasai Heijuro, "out the Outer Mongol intruders from the Olankhuduk region".

After this, they began to withdraw, but were attacked by 5-6 Mongolian armored cars and 2 bombers, which briefly wreaked havoc on a Japanese column.

Local Japanese forces counter-attacked, running dozens of bombing sorties on the village, and eventually assaulting it with 400 men and 10 tankettes.

After incurring several casualties, the Japanese patrol withdrew, and brought up 100 men within hours as reinforcements, who then drove off the Soviets.

In June 1937, the Kanchazu Island incident (乾岔子島事件, Kanchazutou jiken) (ja) occurred on the Amur River at the Soviet–Manchukuo border.

Three Soviet gunboats crossed the center line of the river, unloaded troops, and occupied Kanchazu (also spelled "Kanchatzu") island.

Soldiers from the IJA 1st Division, using two horse-drawn 37mm artillery pieces, proceeded to hastily set up improvised firing sites, and load their guns with both high-explosive and armor-piercing shells.

[7] Soviet-Japanese relations were chilled by the invasion and Mikhail Kalinin, the Soviet head of state, told the American ambassador William C. Bullitt in Moscow that same month that his country was prepared for an attack by Nazi Germany in the west and Japan in the east.

[16][17][18] Later in 1941, Japan considered breaking the pact when Nazi Germany invaded the Soviet Union in Operation Barbarossa but they made the crucial decision to keep it and to continue to press into Southeast Asia instead after the Japanese Attacked on Pearl Harbor .

Four months later, prior to the expiration of the neutrality pact and between the atomic bombings of Hiroshima and Nagasaki, the Soviet Union declared war on Japan, completely surprising the Japanese.