Indeed the invasion of Ukraine is terrible all-round but the UN is already doing what they can to stop it. The UN is not some independent organization, it's member-based - but often it still cops direct criticism when the blame lies with the members.. For example, the UN Security Council can't do much about this war because Russia has veto powers - which is due to the very unfortunate fact that Russia has nuclear weapons and a key aim of the UN is to prevent nuclear war, which it does by trying to create discussion/consensus amongst nuclear powers. Even if Russia's veto was taken away, what could actually be done by the UN further to what is being done now?