The national resistance to the Japanese invasion started on July 7, 1937. There were regional clashes and even wars between Japan and Russia on Chinese land, but the central government had been basically siting aside until July 7, 1937 when Chinese army stationed in Lugou (aka Marco Polo) Bridge were attacked by Japanese army.
It was estimated that more than 30 million Chinese lives were lost in the eight years war from 1937 to 1945. After the then capital city Nanjing was taken by Japanese occupation, about 400,000 residents were slaughtered in a few days. American Chinese historian Iris Chang wrote a book 'Rape of Nanking' about this history (Penguin, 1998).