The Manchukuoan Imperial Army saw some action against the Soviet Union during the Soviet–Japanese border conflicts, which played out largely in the Manchuria region. When the Battle of Lake Khasan occurred in the summer of 1938, Manchukuoan units were mainly used as reserves and saw little fighting, and reportedly some regiments mutinied during the battle. In May 1939, skirmishes broke out between the Mongolian cavalry (Mongolia was an ally of the Soviet Union) and Manchukuoan troops. The battle escalated as both sides brought in reinforcements and after four months of fighting the Battle of Khalkhin Gol ended up being a defeat for the Japanese and Manchukuo. About 18,000 Manchukuoans took part in the battle, mainly cavalrymen of the 7th and 8th Cavalry Regiments. They were initially held in reserve and were sent to the front line in July shortly after being reinforced to divisional strength. These units were positioned on the left flank of the Kwantung Army as it advanced towards the Khalkhin Gol river. The 1st Cavalry Regiment was then sent into battle in the northern sector in August as the situation deteriorated for the Japanese. Although the Manchukuoan units did not perform well overall, the Kwantung Army relied on them due to its desperation for manpower. After the conflict, the Japanese believed that the Manchukuo Imperial Army performed decently and saw reason to expand it.