The Sino-Pakistani relationship has proven remarkably durable during the past fifty years, based as it is on a realistic power calculation, emerging from their mutual enmity toward India. Though Sino-Pakistani relations have developed to accommodate a host of shared concerns, the friendship begins and ends with India. In March 1963, foreign ministers Chen Yi and Zulfikar Ali Bhutto signed the Sino-Pakistan boundary agreement in Beijing. While Chinese foreign relations were largely derailed throughout the Cultural Revolution, Sino-Pakistani ties survived remarkably well, and proved a critical prerequisite for the Sino-American backchannel communication that facilitated Nixon's historic February 1972 visit to the People's Republic of China (PRC). The challenge of transnational terrorism and Islamic fundamentalism has posed a threat to Sino-Pakistani border diplomacy since the mid-1990s. After the 1971 Indo-Pakistani war, Sino-Pakistani relations transitioned from an emphasis on navigating Cold War geopolitical alignments toward bilateral interaction.