"Throughout the nuclear age, the United States, Russian Federation, China, France, and the United Kingdom - the five states permitted by the 1968 Nuclear Nonproliferation Treaty (NPT) to possess nuclear arsenals (the 'NPT nuclear five')- have criticized other nuclear states, or each other, for engaging in dangerous or destabilizing behavior with regard to their nuclear forces. Criticisms have implicitly or explicitly called out offending states for deviating from behavior associated with 'responsible' nuclear states. But what exactly constitutes responsible behavior for nuclear-armed states, and what norms or rules should they follow?"
National Defense University. Center for the Study of Weapons of Mass Destruction: http://www.ndu.edu/WMDCenter/