the People's Republic of China and its allies, while the Warsaw Pact and associated nations, prominently Cuba, gave similar support to ZAPU and its Zimbabwe People's Revolutionary Army (ZIPRA). ZAPU and ZIPRA were headed by Joshua Nkomo throughout their existence, while the Reverend Ndabaningi Sithole founded and initially led ZANU. The two rival nationalist movements launched what they called their "Second Chimurenga" against the Rhodesian government and security forces during the mid-1960s. The army, air force and police successfully repulsed numerous guerrilla incursions, most of which were perpetrated by ZIPRA, over the rest of that decade. After abortive talks between Smith and Wilson in 1966 and 1968, a constitution was agreed upon by the Rhodesian and British governments in November 1971; however, when a British test of Rhodesian public opinion was undertaken in early 1972, black opinion was judged to be against the new deal, causing it to be shelved. The Rhodesian Bush War suddenly re-erupted in December 1972, after two years of relative inactivity, when ZANLA attacked Altena and Whistlefield Farms in north-eastern Rhodesia. After a successful security force counter-campaign during 1973 and 1974, drastic changes in the foreign policy of the Rhodesian government's two main backers, Portugal and South Africa, caused the conflict's momentum to shift in the nationalists' favour. In April 1974, the Portuguese government was overthrown by a military coup and replaced with a leftist administration in favour of ending the unpopular Colonial War in Angola, Mozambique and Portugal's other African territories. The institution by Pretoria of a détente initiative in late 1974 forced a ceasefire in Rhodesia, and in June 1975 Mozambique became independent from Portugal under a communist government allied with ZANU. Unsuccessful rounds of talks were held between the Rhodesian government and the nationalists, united under the banner of Abel Muzorewa's African National Council, across the Victoria Falls Bridge in August 1975, then directly between the government and ZAPU starting in December 1975. Around this time, Robert Mugabe replaced Sithole as ZANU leader, winning an internal leadership election which Sithole refused to recognise. Guerrilla incursions picked up strongly in the first months of 1976, leading Smith to declare on the evening of 6 February 1976 that "a new terrorist offensive has begun and, to defeat it, Rhodesians will have to face heavier military commitments." Security force reports indicated that around 1,000 insurgent fighters were active within Rhodesia, with a further 15,000 encamped in