OBAMA THREATENS FORCE AGAINST IRAN OVER BOMB
U.N. charter prohibits both threats and force

"Barack Obama has delivered his starkest threat so far over Iran's nuclear ambitions, saying America is ''not bluffing'' when it warns military action could be used to stop Tehran acquiring the bomb.
http://www.smh.com.au/world/us-threatens-iran-on-bomb-20120303-1u9kn.html

But the U.N. Charter, to which the U.S. is a signatory, explicitly prohibits not just a military attack on another nation, but also the issuance of threats of such an attack.
"All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations."
--United Nations Charter, Chapter I, Article 2, Paragraph 4
http://www.un.org/en/documents/charter/chapter1.shtml