The United Nations was established in 1945 to replace the ineffective League of Nations and promote international cooperation. It has six main organs: the General Assembly, Security Council, Secretariat, Economic and Social Council, International Court of Justice, and Trusteeship Council. The UN works to maintain peace and security, protect human rights, deliver humanitarian aid, and achieve sustainable development goals. It faces ongoing challenges in peacekeeping, human rights, disputes, poverty, and threats like terrorism.