You seem to be confusing "Anticipatory" self-defense with Preemtive War.. We can Defend ourselves against a Nation when an attack is immenint, We can also defend ourselves from Terrorists in the same respect. But, we can Not launch a "War on Terror" as to be a WAR, Congress has to declare a War against a Nation, Constitutionally speaking, "war" is a very specific set of legal relations between two or more independent nations. For the most obvious example, in an actual "war" soldiers of one nation may, within certain limits, intentionally kill soldiers of another nation without thereby being guilty of murder. Thus, according to strict constitutional logic, a "war on terror" is an existential impossibility--if only because "terror" is a tactic, not a country; and "terrorists" do not constitute one or more independent nations, but at most are mere bands of private criminals.
Article VI of the U.S. Constitution establishes that ratified treaties, such as the U.N. Charter, are the "supreme law of the land." Now you can Google The Article 1 and 2 of the UN Charter..
We can Not pick and choose what parts of the Constitution to support, if we want the Second amendment, we have to take it ALL, even if we do not totally agree with what it entails. Allowing the President, Congress, or anyone else to Ignore parts of it is reckless... And we already see what it leads to.