In mid-1937, following the Marco Polo Bridge Incident, Japan began a full invasion of China. The Soviets quickly lent support to China, effectively ending China’s prior cooperation with Germany. Starting at Shanghai, the Japanese pushed Chinese forces back, capturing the capital Nanjing in December. In June of 1938 Chinese forces stalled the Japanese advance by flooding the Yellow River. Though this bought time to prepare their defenses at Wuhan, the city was still taken by October. During this time, Japanese and Soviet forces engaged in a minor skirmish at Lake Khasan; in May of 1939, they became involved in a more serious border war.
In Europe, Germany and Italy were becoming bolder. In March 1938, Germany annexed Austria, again provoking little response from other European powers. Encouraged, Hitler began making claims on the Sudetenland; France and Britain conceded these for a promise of no further territorial demands. Germany soon reneged, and in March 1939 fully occupied Czechoslovakia.
Alarmed, and with Hitler making further demands on Danzig, France and Britain guaranteed their support for Polish independence; when Italy conquered Albania in April, the same guarantee was extended to Romania and Greece. The Soviet Union also attempted to ally with France and Britain, but was rebuffed due to western suspicions about Soviet motives and capability. Shortly after the Franco-British pledges to Poland, Germany and Italy formalized their own alliance with the Pact of Steel; following this, in a move that shocked all other major powers, Germany and the Soviet Union concluded a non-aggression pact, including a secret agreement to split Poland and eastern Europe between them.
By the start of September the Soviets had routed Japanese forces and the Germans invaded Poland. France, Britain and the Commonwealth declared war on Germany but lent little support other than a small French attack into the Saarland. In mid-September, after signing an armistice with Japan, the Soviets launched their own invasion of Poland. By early October, Poland had been divided between Germany and the Soviet Union. During the battle in Poland, Japan launched its first attack against Changsha, a strategically important Chinese city, but was repulsed by early October.