After the Xinhai Revolution (1911–12) toppled the Qing dynasty and the last Qing troops were escorted out of Tibet, the new Republic of China apologized for the actions of the Qing and offered to restore the Dalai Lama's title. The Dalai Lama refused any Chinese title and declared himself ruler of an independent Tibet. In 1913, Tibet and Mongolia concluded a treaty of mutual recognition. For the next 36 years, the 13th Dalai Lama and the regents who succeeded him governed Tibet. During this time, Tibet fought Chinese warlords for control of the ethnically Tibetan areas in Xikang and Qinghai (parts of Kham and Amdo) along the upper reaches of the Yangtze River. In 1914 the Tibetan government signed the Simla Accord with Britain, ceding the South Tibet region to British India. The Chinese government denounced the agreement as illegal.
What did the Tibetan government sign in 1913?