The Chinese resistance stiffened after 7 July 1937, when a clash occurred between Chinese and Japanese troops outside Beijing (then named Beiping) near the Marco Polo Bridge. This skirmish led to open, though undeclared, warfare between China and Japan. Shanghai fell after a three-month battle during which Japan suffered extensive casualties, both in its army and navy. The capital of Nanjing fell in December 1937. It was followed by an onslaught of mass murders and rapes known as the Nanjing Massacre. The national capital was briefly at Wuhan, then removed in an epic retreat to Chongqing, the seat of government until 1945. In 1940 the collaborationist Wang Jingwei regime was set up with its capital in Nanjing, proclaiming itself the legitimate "Republic of China" in opposition to Chiang Kai-shek's government, though its claims were significantly hampered due to its nature as a Japanese puppet state controlling limited amounts of territory, along with its subsequent defeat at the end of the war.