The Chinese city of Nanjing is marking the 70th anniversary of one of the worst atrocities of the second world war in Asia. On December 13, 1937, Japanese troops entered what was then the capital of China's nationalist Kuomintang government, beginning a campaign of mass murder and rape that lasted for six weeks. The exact number who died in what became known as the Rape of Nanjing remains in dispute. China says up to 300,000 were killed, many of them women and children.