Depends how you define "legal measure", haha. Generally, one would argue those above IGOs (UN, EU) are legal measures since they are based around fundamental principles that outline how countries and governments should behave to preserve the rule of law and ensure human rights are maintained around the world, after the devastation of World War II, especially the UN. Although one could then argue how successful these organisations have been and there failures in recent history to maintain order and prevent conflict but that is for another day. Essentially, it is the UN that enables the existence of "International Law" and without the United Nations or a body like it an international legal order would be essentially impossible to maintain. The EU and NATO are quite different beasts. The EU is more economical in nature, a large "free trade zone" if you will, although there are certain aspects surrounding human rights, government integrity and harmonised legal systems which play a role. Finally NATO is essentially a defence pact/alliance wrapped up under the pretext of promoting "democracy" which is essentially just a vehicle for the US to maintain its hegemony over Canada and Western Europe. Anyway, hope that helps!