This is a great op-ed, tracing the spread of nuclear technology from Nazi Germany to the former USSR and Pakistan. Looking at today's problems, it is really too bad that no one in the early 1990s followed Kerry's recommendation to investigate how Pakistan subverted the non-proliferation laws. It's amazing that both the mainstream Democrats and the Republicans have vested interests in keeping the fact that Kerry strongly advocated for investigating this. (An investigation could have missed A. Q. Khan's network, but it wasn't even initiated.)
"This is not the place to go into the details of Dr. Khan’s activities, which in the end involved a variety of countries from Libya to China — to say nothing of Iran, whose centrifuges also have a Pakistani origin.
By the 1990s Dr. Khan was exchanging weapons information with the North Koreans for similar information about their long-range rockets. We know he gave them plans for the centrifuge and probably sample centrifuges. We do not know whether he gave them plans for a nuclear weapon, as he had done for the Libyans.
We also do not know to what extent the government of Pakistan was complicit in this. The army certainly was, and military aircraft were used to transport material. Pakistan has denied any involvement; Dr. Khan is under house arrest and no foreign intelligence representatives have been allowed to interview him."
http://www.nytimes.com/2007/03/10/opinion/10bernstein.html?pagewanted=2&_r=1