One important factor the Czar omits in the excellent post above is that since Partition, almost all attempts to forge a pan-Pakistani identity have focused on the common denominator of religion: the large majority of Pakistanis are Muslim, and the country was theoretically founded as an entity for the Muslims of British India (though there are more Muslims in India than Pakistan to this day).
The stoking of sectarian passions, then, has been a necessary concomitant of Pakistani nationalism in much the way, say, that anti-Albanianism in Kosovo was Slobodan Milošević’s first attempt at forming a post-Communist Yugoslav nationalism—which eventually became the Serbian nationalism which fueled much of the initial stages of the Civil War there (stoking, therefore, Croatian and creating Bosnian nationalisms).
This is not to say that the Islamic “land of the pure” (unlike those unclean infidels with their idols to the southeast) national identity has succeeded in unifying the country—it largely hasn’t. It does, however, explain why Pakistani mobs are always among the most visibly and violently agitated at any putative insult to Islam—they’re encouraged by the government.
In recent history, Zia ul-Haq of odious memory was the most recent fervent Islamic-nationalist leader of Pakistan, and the creation of the Afghan Taliban and Islamist-takeover of the ISI were all under his watch. Moreover, oceans of Saudi money supporting the anti-Soviet jihâd and Waḥḥâbî-style Islam as well as the influence of the local Deobandi school of Islam (very much akin to Waḥḥâbism) have pushed Pakistani Islamist ideology in an increasingly radical direction since the 1980s. America, trying to be a good non-imperialist power, subcontracted our post-Soviet Afghanistan policy to this Cold War ally (remember, India was a Soviet client state) with the unfortunate results that continue to this day.
While Pakistanis are often exceptionally interesting, intelligent, and sophisticated people, the political entity of Pakistan—armed with nukes, perpetually aggrieved in an attempt to distract from internal tensions, and apparently fomenting terrorism almost on the scale of Iran—may end up judged the single most-mistakenly created “nation” of the twentieth century—and that covers a vast and deep array of errors.