The Beeb has some neat maps of the Pakistan situation here.
Looking at them, the thought occurred: What a dumb place to locate your capital city! I wikied the place. Turns out Islamabad is, like Brasilia, a deliberately-created capital city. Like many U.S. state capitals, too, it is far from being the largest city in its territory.
Quote from Wikipedia:
As Capital of Pakistan
When Pakistan was created in 1947, Karachi was the first capital. However, in 1960 Islamabad was constructed as a forward capital due to following reasons:
• Traditionally, the development was focused on the colonial centre of Karachi and President Ayub Khan wanted it to be equally distributed.
• Karachi was located at one end of the country and a capital which was easily accessible from all parts of the country was needed.
• Karachi was vulnerable to attacks from the sea while Islamabad, by contrast, is 750 miles inland and surrounded by mountains.
• A statement was needed to be made regarding Kashmir territories in the North, which were disputed with India.
• It was also closer to the GHQ which was, and still is, in Rawalpindi.
• The climate in Islamabad is favourable compared to Lahore.
I tally those six reasons as: political, political**, military, political, military, climatic. To the degree that Pakistan is really a nation (boy, do my former countrymen have a lot to answer for!) its central organizing principle seems to be conflict with India.
** If you’re thinking “How do you offset the disadvantage of the capital being at one end of the country by moving it to the other end?” note that until 1971 Pakistan was in two noncontiguous pieces, the other piece being present-day Bangladesh.