Kanchazu Island incident

The occurred in late June 1937 on the Amur River (Soviet–Manchukuo border).
Background.
Kanchazu (also spelled "Kanchatzu" ) Island is an unoccupied island. It is located in the center of the Amur River that acted as the official border between Manchuria and the USSR.
Events.
On June 19, two Soviet motorboats crossed the center line of the river, unloaded twenty troops and occupied Kanchazu Island.
On June 20, 17 Manchurian police and soldiers were sent to investigate the border intrusion. Soviet troops, numbering around 40 men, were now entrenched on Kanchazu Island and building fortifications. The Manchurian patrol was driven back by Soviet soldiers.
On 29 June, a planned operation by the Imperial Japanese Army's (IJA) 1st Division headquarters was approved for a night attack on Kanchazu for the expulsion of Soviet troops from the island. The operation was ultimately delayed and rescheduled for the following day.
On the morning of June 30, IJA soldiers from the 49th Regiment of the 1st Division led by Colonel Mihara Kanae launched a prolonged attack against the Soviets. The attack began with the use of two horse-drawn 37 mm artillery pieces. The Japanese soldiers proceeded to hastily set up improvised firing sites, and load their guns with both high-explosive and armor-piercing shells. The shelling was successful in sinking the lead gunboat, killing seven crew members, and crippling the second, and driving off the third. During the confrontation the Soviets responded with minimal return gunfire that did not result in any Japanese casualties.
Soviet crewmen of the first sunken gunboat were left stranded and forced to swim to the north side of the bank into Soviet territory. Many casualties were inflicted when Japanese troops opened fire on the swimming crewmen with a barrage of machine-gun fire. Around 37 Soviet troops were killed in this incident. The island was left abandoned and was reclaimed afterwards by IJA troops.
Aftermath.
Shigemitsu Mamoru, the Japanese ambassador to the Soviet Union, met with Commissar of Foreign Affairs Maxim Litvinov on June 29 to discuss the turn of events. The Soviets insisted the Amur islands belonged to them following an 1860 agreement and their placement on a Soviet map. During negotiations, the Soviets, however, agreed to pull back their forces from the Amur river to defuse the situation. Apparently, the Soviets were more concerned with events unfolding in North China and Europe, as well as internal strife. In the aftermath, seven Soviet gunboats appeared sometime in July, but the Japanese took no action.
Eventually, as part of the agreement the Soviets were allowed to salvage the sunken gunboat, which was accomplished between October 22–29 in the same year.