With regard to war, international and constitutional laws are clear. Under the Constitution's Article I, Section 8, only Congress may declare war, not the president. That, in fact, last happened on December 8, 1941 after Japan attacked Pearl Harbor. As a result, all subsequent US wars have been illegal, including Obama's against Iraq, Afghanistan, Pakistan and Libya.
Moreover, the UN Charter explains under what conditions violence and coercion (by one state against another) are justified.
Article 2(3) and Article 33(1) require peaceful settlement of international disputes. Article 2(4) prohibits force or its threatened use. And Article 51 allows the "right of individual or collective self-defense if an armed attack occurs against a Member....until the Security Council has taken measures to maintain international peace and security."
In other words, justifiable self-defense is permissible. However, Charter Articles 2(3), 2(4), and 33 absolutely prohibit any unilateral threat or use of force not:
