Thirty Years' War, there was an outcry for rules of warfare to protect civilian populations, as well as a desire to curb invasions. The League of Nations, established after the war, attempted to curb invasions by enacting a treaty agreement providing for economic and military sanctions against member states that used "external aggression" to invade or conquer other member states. An international court was established, the Permanent Court of International Justice, to arbitrate disputes between nations without resorting to war. Meanwhile, many nations signed treaties agreeing to use international arbitration rather than warfare to settle differences. International crises, however, demonstrated that nations were not yet committed to the idea of giving external authorities a say in how nations conducted their affairs. Aggression on the part of Germany, Italy and Japan went unchecked by international law, and it took a Second World War to end it. The postwar era After World War II, as after the First World War and the Thirty Years' War, there was a strong desire to never again endure the horrors of war endured by the civilian populations. The League of Nations was re-attempted through another treaty organization, the United Nations. The postwar era has been a highly successful one for international law. International cooperation has become far more commonplace, though of course not universal. Importantly, nearly two hundred nations are now members of the United Nations, and have voluntarily bound themselves to its charter. Even the most powerful nations have recognized the need for international cooperation and supports, and have routinely sought international agreement and consent before engaging in acts of war. International law is, of course, only partly about the conduct of war. Most rules are civil, concerning the delivery of mail, trade, shipping, air travel, and the like. Most rules are obeyed routinely by most countries, because the rules make life easier for all concerned. The rules are rarely disputed. But some international law is extremely political and hotly debated. This includes not just the laws of warfare but also such matters as fishing rights. Modern customary international law An important development in modern international law is the concept of "consent." Before World War II, a nation would not have been considered to be bound by a rule unless it had formally agreed to be bound by it, or it was already customarily abiding by that rule. Now, however, merely consenting to an international