An ally, in one perhaps cynical definition, is someone you can't trust enough to call a friend. Pakistan has been that ally since it's inception. Pakistan was largely the creation of one man, Muhammed Ali Jinnah, and its rationale for existence was to prevent Muslims from having to live in a country with a Hindu majority. In that respect, as in many, it has failed, with the Muslim population of the subcontinent now divided among three countries: India, Pakistan, and Bangladesh, with with Pakistan having roughly 2/5 of the total.
Husain Haqqani, scholar, journalist, and former Pakistani ambassador to the US, has written an history of US Pakistan relations, and its title could also be its summary: Magnificent Delusions: Pakistan, the United States, and an Epic History of Misunderstanding
Both the US and Pakistan have been seriously deluded in what they have expected from each other, but Pakistan probably takes the bigger hit in the early going. Margaret Bourke-White, the Life magazine journalist, had a long interview with Jinnah in the earliest years of Pakistan's existence, and she came away very unimpressed. Haqqani on her conclusions:
Jinnah’s expectation of US aid for Pakistan, American officials’ concerns about anti-Americanism, and Bourke-White’s cynicism about Pakistani objectives around the time of the country’s inception together seem like the prologue to a story with many repetitions. The Life correspondent discerned in Pakistan a persistently voiced “hope of tapping the US treasury,” which led her to wonder “whether the purpose was to bolster the world against Bolshevism or to bolster Pakistan’s own uncertain position as a new political entity.”
Ultimately, in Bourke-White’s opinion, “it was more nearly related to the even more significant bankruptcy of ideas in the new Muslim state— a nation drawing its spurious warmth from the embers of an antique religious fanaticism, fanned into a new blaze.” 4
Haqqani, Husain (2013-11-05). Magnificent Delusions: Pakistan, the United States, and an Epic History of Misunderstanding (pp. 10-11). PublicAffairs. Kindle Edition.
Haqqani is a Pakistani patriot, but he has been exiled from his own country for his insistence that its political direction and popular mood is seriously misguided. He notes that nearly all US allies in post World War II world have prospered greatly, but Pakistan is a lonely exception.