Pakistan was originally signed up to a civilian programme, but after war with India and Indian nuclear testing decided they needed nuclear weapons following a predominently second strike doctrine. However, Pakistan's position is that in the event of invasion they will respond immediately with nuclear weapons[1].
They received substantial help from the Chinese, and had nuclear reactors at the time the programme started in the 70s (under the codename Project 706[2]). They're not signed up to the NPT or CTBT either (choosing to sign neither until India do).
A new book has just been released on the history of Pakistan's nuclear program: Eating Grass: The Making of the Pakistani Bomb by Feroz Khan (http://www.sup.org/book.cgi?id=21098).
It bears pointing out that the decision to build a nuclear weapon is at least as much a political one as a technical one.
Pakistan has operated nuclear reactors since the 1950s and they undertook a nuclear weapons development program starting in the 1970s and reaching fruition in the late 80s. They already had plentiful sources of weapons grade Plutonium and through what should properly be called espionage they gained access to advanced technologies in nuclear fuel reprocessing, Uranium isotope separation, and nuclear weapon design.
