Nations have always formed alliances. In the 20th century, many new international organizations were set up worldwide for economic and political reasons, defense and peacekeeping, and health and welfare.
The League of Nations was founded in 1919 to keep the peace after World War I. But it failed to prevent World War II in 1939, so in 1945 the nations of the world set up a new organization, the United Nations (UN). Since then, the UN has encouraged international cooperation and worked to resolve conflict.
Some alliances have been political, such as the Arab League (1945) or the Organization of African Unity (1963). The European Economic Community (1958) grew from a small trading alliance into the European Union. Military alliances included the North Atlantic Treaty Organization (1949–) and the Warsaw Pact (1955–1991).