International law and the United Nations go hand in hand.
However, there is the issue of pre-emptive strikes. I know that this is the issue raised by international law. International law is evolving and so it should. If there had been pre-emptive strikes on Hitler, from 1933 to 1936, before Munich in 1937, before the Sudetenland crisis, before the Anschluss, before the invasion of Poland in 1939, this would have been good for all humankind, even if international law did not allow it. That said, we still need evidence. We had sufficient evidence against Adolf Hitler. The pacifists were wrong not to act. Many were not able to look back after 1945 because they were gone. We still need evidence and, I repeat, right now, there is no such evidence.