Firstly, it is important to point out that nuclear weapons are terror weapons. They have limited military value and are design to kill large numbers of civilians.

One of the problems is that deterrence does not work if you do not know who attacked you. It could be a non-state player or a state which hides the source of an attack to avoid retaliation.

A simple ‘gun type’ nuclear weapon could be made very easily if the correct amount of highly enriched uranium (HEU) was available. There have been numerous attempts to get such material, I am not going to go into further detail since this has been written about extensively elsewhere¹.

The U.S. has not always been that careful about tracking HEU which it has exported to other countries and the authorities:

“were able to verify the location of 1,160 kilograms out of 17,500 kilograms of U.S. HEU estimated to have been exported”²

• Nuclear Weapons: Keeping the peace for 60 years?
• Nuclear Weapons: Ending The War With Japan
• Nuclear Deterrence: Cuban Missile Crisis
• Deterrence: Terrorists

¹ Usable Nuclear Materials Worldwide Must be Protected Against Global Terrorist Threats, Matthew Bunn and Col-Gen. E.P. Maslin (http://belfercenter.ksg.harvard.edu/files/all-stocks-of-weapons-usable-material-eng.pdf)

² NUCLEAR NONPROLIFERATION: U.S. Agencies Have Limited Ability to Account for,Monitor, and Evaluate the Security of U.S. Nuclear Material Overseas U.S. Government Accountability Office (GAO) (http://gao.gov/assets/330/323054.pdf)