Portugal attempted to assert its African territorial claims through three expeditions led by Alexandre de Serpa Pinto, first from Mozambique to the eastern Zambezi in 1869, then to the Congo and upper Zambezi from Angola in 1876 and lastly crossing Africa from Angola in 1877–1879. These expeditions were undertaken with the intention of claiming the area between Mozambique and Angola. Following Serpa Pinto's explorations, the Portuguese government in 1879 made a formal claim to the area south and east of the Ruo River (the present south-eastern border of Malawi) and, in 1882, occupied the lower Shire River valley as far as the Ruo. The Portuguese then asked the British government to accept this territorial claim, but the opening of the Berlin Conference of 1884–85 ended the discussions. Portugal's efforts to establish this corridor of influence between Angola and Mozambique were hampered by one of the articles in the General Act of the Berlin Conference which required effective occupation of areas claimed rather than historical claims based on discovery or those based on exploration, as Portugal had used.