No country has an inherent right to invade or use aggression against another.  Given the moral bankruptcy of the NPT, and existing views of the United States in much of the developing world, [1] any move by the United States to prevent other nations from developing nuclear weapons by force will be seen for what it is: an act of neo-colonialism. This would be the case with any act to enforce a treaty that is considered unfair towards most of the world.  This is especially true in areas where there is a long history of US support for regional actors who are less than popular. In moving against Iran, the United States will be perceived as a stalking horse for Israel, whilst any efforts to invade North Korea Would cause great alarm in China as well as in neighbouring South Korea despite being a U.S. ally where some Koreans believe the US is more of a threat to the nation than the North. [2] In both cases, the image of the US in the region will be badly damaged, and the United States will face a hostile insurgency within the countries that they invade.  [1] Pew Global Attitudes Project, 2011,   [2] Larson, Eric V. et al., Ambivalent Allies? A Study of South Korean Attitudes Towards the U.S., RAND Corporation, March 2004, p.93  (n.b. before north detonated nuclear bomb)