Habibullah was assassinated during a hunting trip in February 1919, and Amanullah Khan eventually assumed power. A staunch supporter of the 1915–1916 expeditions, Amanullah Khan provoked the Third Anglo-Afghan War, entering British India via the Khyber Pass. After the end of the Third Anglo-Afghan War and the signing of the Treaty of Rawalpindi on 19 August 1919, Emir Amanullah Khan declared the Emirate of Afghanistan a sovereign and fully independent state. He moved to end his country's traditional isolation by establishing diplomatic relations with the international community, particularly with the Soviet Union and the Weimar Republic of Germany. He proclaimed himself King of Afghanistan on 9 June 1926, when the Emirate of Afghanistan became the Kingdom of Afghanistan. Following a 1927–28 tour of Europe and Turkey, he introduced several reforms intended to modernize his nation. A key force behind these reforms was Mahmud Tarzi, an ardent supporter of the education of women. He fought for Article 68 of Afghanistan's 1923 constitution, which made elementary education compulsory. The institution of slavery was abolished in the Emirate of Afghanistan in 1923. King Amanullah's wife, Queen Soraya, was an important figure during this period in the fight for woman's education and against their oppression. Some of the reforms that were put in place, such as the abolition of the traditional burqa for women and the opening of several co-educational schools, quickly alienated many tribal and religious leaders, and this led to the Afghan Civil War (1928–1929). Faced with the overwhelming armed opposition, King Amanullah abdicated in January 1929, and soon after Kabul fell to Saqqawist forces led by Habibullah Kalakani. Prince Mohammed Nadir Shah, Amanullah's cousin, in turn defeated and killed Kalakani in October 1929, and was declared King Nadir Shah. He abandoned the reforms of King Amanullah in favor of a more gradual approach to modernization, but was assassinated in 1933 by Abdul Khaliq, a fifteen-year-old Hazara student who was an Amanullah loyalist. Mohammed Zahir Shah, Nadir Shah's 19-year-old son, succeeded to the throne and reigned as King from 1933 to 1973. The tribal revolts of 1944–1947 saw King Zahir's reign challenged by Zadran, Safi, Mangal, and Wazir tribesmen led by Mazrak Zadran, Salemai, and Mirzali Khan, among others, many of whom were Amanullah loyalists. Close relations with the Muslim states Turkey, the Hashemite Kingdom of Iraq and Iran/Persia were also pursued, while further international relations were sought by joining the League