Historical alliances are agreements between countries or groups to support each other, often for military or political purposes. These alliances can shape the course of history by influencing wars, trade, and diplomacy. Notable examples include the Triple Alliance of Germany, Austria-Hungary, and Italy before World War I, and the Allied Powers during World War II, which included countries like the United States, United Kingdom, and Soviet Union. Alliances can be formal, with treaties and written agreements, or informal, based on mutual interests. They often arise in response to perceived threats or to enhance security. The NATO alliance, formed in 1949, is a modern example, promoting collective defense among its member nations.