It is hard to fathom what the Nuclear conference in Washington is designed to achieve other than a photo op. There is not need to gather so many world leaders to discuss Iran as the UN Security Council will decide on whether to extend sanctions. Anyway Iran may become a threat to Israel but is not likely to be a threat to the US for a long long time. As for terrorist groups , it is laughable to think that any of these bunch of fanatics will have the knowledge or the materials to build a nuclear bomb and then be able to smuggle it into the US. Plutonium and enriched uranium are extremely radioactive and careless handling will result in severe burns and even death. There is no way anyone will be able to smuggle in enough plutonium and the various parts required to build a bomb into the US without being caught in seconds. As for a dirty bomb there is enough medical waste inside the US to make one but its range will be very restricted. Only a national agency can build a nuclear weapon small enough to be transported by, say, a boat and then be able to put together a team to carry out an attack. The 9/11 attacks were organised inside Pakistan and a nuclear attack is most likely to originate there. The civilian government has no control over the army, the ISI, the Taliban and other terrorist groups. Indeed the parliament is busy reducing the powers of the President and trying to bring Sharif back to power which may precipitate a power struggle. There are enough fanatic, lunatic elements within the ISI to want to attack the US again. Look out for a fast motor boat which drops from an ordinary cargo ship a makes at full speed towards a small town along the coast.