Pakistan has been facing security threats since its independence, and looking for security assurances from external sources. Its alignment policy with the U.S. has been primarily based on its heightened sense of insecurity at different times. This sense of insecurity and the U.S. need for allies brought the two countries together during the Cold War. Military collaboration has been an important component of this alliance. Since then, the U.S.-Pakistan security relationship has been a roller coaster ride with periodic highs and lows at both ends. This relationship has been based on periods of friendship as well as mutual distrust. Pakistan and the U.S. developed new security strategies in the region to face the growing threat from Afghanistan in the 1980s and after 2001. American economic aid and the arms sales packages were meant to meet Pakistan’s security needs during the Cold War and the Afghanistan Jihad. This was also meant to promote U.S. interests in the region. The U.S. has enforced a reactive policy towards Pakistan by relating aid to other requirements, such as nuclear nonproliferation, strengthening of democratic institutions and to fight terrorism. After September 11, 2001, Pakistan and the U.S. once again formed a security partnership. Pakistan by virtue of its geopolitical environment became a natural and frontline ally in the U.S led global War on Terror. However, Pakistan’s role in this war is a complex phenomenon in terms of its implications. Though, Pakistan was an isolated country prior to 9/11 due to nuclear sanctions, military takeover by General Musharraf, and close ties with the Taliban regime in Afghanistan. Pakistan once again became a U.S. ally to fight terrorism. This new relationship brought new opportunities and challenges for the two allies. While the War on Terrorism has made Pakistan to pay a heavy price, it has managed to build a sustainable 10 relationship with the U.S. However, the future remains uncertain pertaining to the conclusions of the War on Terror, and future developments in Afghanistan. This dissertation analyzes the deciding factors in the Pakistan-U.S. relationship in the post 9/11 era. While the main focus remains on the complex dynamics of the relations, and Pakistan’s role in the War on Terror is highlighted.