Pakistan-US relations evolved in the cold war perspective. Pakistan‟s economic and national security needs and United States‟ need of allies for containment of communism and strategic compulsions in South Asian Region forged their strategic relationship. Bilateral interests of both the countries defined their mutual relations as well as depth of these relations for a short or long term. Pakistan was in search of allies due to its fear or threat perception of India. For US, Pakistan became an ideological ally during expansion of communism as it was a common and the biggest threat after the Second World War to America. Pakistan-US relations have many dimensions in their historical perspective and are like travelling on a bumpy road marred with mistrust and disenchantment. However, it is interesting to study and analyse Pakistan-US bilateral relations so that some logical conclusions may be drawn for assessing theoretical basis and significance. There is no such study in the field which had analyzed the impact of fear and threat perception for guiding foreign policy behaviour of Pakistan and its implications for national security. Pakistan-US relations worked in traditional realist paradigm with neoclassical realist approach studying structural changes in South Asian Regional Security Complex as national security remained paramount consideration for Pakistan to maintain a balance of power with India. Economic and strategic insecurity perceived due to fears of Indian hegemony and threats to sovereignty and national security of Pakistan always led its decision-making. Security dilemma is applied to analyze proportionality and impact of fear or threat perception in international behaviour for maintaining balance of power. The study aims to establish a theoretical framework for state behaviour in fear and insecurity in anarchical perspective of international relations purely for national security and interests beneficial for other weaker states in their relationship with great powers. There is no permanence in state relations and domestic, regional and global dynamics shape the behaviour of states. The study reveals convergences and divergences in bilateral relations. Mutual interests dictated convergence and afterwards US never taken care of Pakistan whenever its interests were served leaving Pakistan in the lurch frustrated and disenchanted. Pakistan learnt lessons in international behaviour in realist paradigm where only interests matter as there are no permanent friends and foes.