Positioned at a geo-politically sensitive crossroads, Pakistan has since the 1950s served off and on as a "cornerstone of US policy" in the Middle and Near East, joining the South East Asia Treaty Organisation (SEATO) in 1954. With Iraq opting out in 1958, the US-sponsored "Baghdad Pact" (1955) became the now-defunct (1979) Central Treaty Organisation (CENTO) comprising non-Arab Muslim countries Turkey, Iran and Pakistan. The economic equivalent of CENTO, the Regional Cooperation for Development (RCD) turned into the Economic Cooperation Organisation (ECO), eventually growing to eight countries. ECO's growth was retarded by the sanctions against Iran.
Once the US need for our services ended after the Soviet evacuation from Afghanistan in 1989, our relations went downhill when a number of reasons from the early 80s were revived, (1) our suspect nuclear ambitions (2) a haven for drugs manufacturing and smuggling thereof and (3) suspected ISI support for terrorist activity (we narrowly escaped being branded a terrorist state in 1992). Beginning the 90s decade we were an ally; this relationship had undergone a 180 degree turnaround by the time we detonated the nuclear explosion at Chagai on May 28, 1998, sanctions thereafter were mandatory. The revival of the "cornerstone" status again after 9/11 was not surprising; we were needed as the platform for the US war in Afghanistan. The short telephone call from the then US Secretary of State, General Colin Powell, to General Musharraf got the US our "services" fairly cheap!
The motivated "tilt" towards Pakistan every other decade notwithstanding, there is no such ambiguity about the US now being firmly behind India. Throughout the Cold War, India vociferously supported the Soviet Union, receiving military hardware exceeding that to any of its Warsaw Pact allies, many times more than US military aid to Pakistan. India vehemently opposed the 80s Afghan war against the Soviets in Afghanistan.
Ambassador Chester Bowles' Memo describes his conversations encouraging a rather reluctant Indian PM Jawaharlal Nehru during his visit to New Delhi on August 8 and 9, 1961 to flex India's military muscle beyond its immediate neighbourhood to "contain communist China". To quote US Ambassador to India, Chester Bowles letter to President Lyndon B Johnson on April 14, 1965, "our present military alliance with Pakistan has become irrelevant to the present situation in Asia, this policy was created in a different era to meet totally different conditions. India by virtue of it size, resources, potential and economic potential and geographic location is of great importance to US national interest. India's domestic and foreign objectives coincide more closely with our (ie US) interests than do these of any other major emerging nation "for meeting the threat from Communist China." Chester Bowles was a key formulator of the so-called "Asian Monroe Doctrine" to extend India's domination over the Indian Ocean, its neighbouring States and South East Asia. Despite the US mostly air-lifting to Calcutta arms and equipment according to the Indian "wish list" for four "mountain" divisions during India's short China War in 1962 (even the Americans balked at the Indians asking for submarines to fight the Chinese in the Himalayas), India remained firmly aligned behind the Soviet Union in all world forums much beyond the final collapse of the USSR in 1991.
India's "marriage" with the US would have been consummated had it not been for 9/11. Forced to turn again to Pakistan, the Indian "bride" was left jilted at the altar by the US. In US diplomatic parlance "India is an actor to the US pivot to Asia-Pacific", it is the "containing of China" plan in more blunt language, promoted in earnest by Barack Obama since 2008, getting added traction after Narendra Modi came to power in 2014. Were there any doubts left in Pakistani minds about fulfilling Chester Bowles' dream of supporting India as an American ally and regional super-power after the American President's Chief Guest appearance at the Republic Day "Armed Forces Parade" in New Delhi? This has now been finally "formalized" by Obama. The US support for India joining the Nuclear Suppliers Group (NSG) while refusing Pakistan shows that the regional exceptionality when dealing with nuclear Pakistan is not a restraining factor for the US anymore.
To quote my article