ノモンハン事件
Japanese expansion in the Northeast China region bordering the Soviet Far East and disputes over the demarcation line led to growing tensions with the Soviet Union, with both sides often violating the border and accusing each other of border violations. 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 Battles of Khalkhin Gol in 1939 which resolved the dispute and returned the borders to "status quo ante bellum".