WHY THE U.N. CHARTER NO LONGER CONTROLS ON USE OF FORCE: Because it's been a disastrous failure and nobody follows it. Here's a quote:
International "rules" concerning use of force are no longer regarded as obligatory by states. Between 1945 and 1999, two-thirds of the members of the United Nations--126 states out of 189--fought 291 interstate conflicts in which over 22 million people were killed. This series of conflicts was capped by the Kosovo campaign in which nineteen NATO democracies representing 780 million people flagrantly violated the Charter. The international system has come to subsist in a parallel universe of two systems, one de jure, the other de facto. The de jure system consists of illusory rules that would govern the use of force among states in a platonic world of forms, a world that does not exist. The de facto system consists of actual state practice in the real world, a world in which states weigh costs against benefits in regular disregard of the rules solemnly proclaimed in the all-but- ignored de jure system. The decaying de jure catechism is overly schematized and scholastic, disconnected from state behavior, and unrealistic in its aspirations for state conduct.
The upshot is that the Charter's use-of-force regime has all but collapsed. This includes, most prominently, the restraints of the general rule banning use of force among states, set out in Article 2(4). The same must be said, I argue here, with respect to the supposed restraints of Article 51 limiting the use of force in self-defense. Therefore, I suggest that Article 51, as authoritatively interpreted by the International Court of Justice, cannot guide responsible U.S. policy-makers in the U.S. war against terrorism in Afghanistan or elsewhere.
This is from a law review article by Prof. Michael Glennon, The Fog of Law: Self-Defense, Inherence, and Incoherence in Article 51 of the United Nations Charter, 25 Harvard Journal of Law and Public Policy 539 (2002). (Emphasis added above).