Alliance--alliances are agreements to help in case of war or economic need but also, when known, will deter a country from attacking another knowing they will have to fight multiple nations.
Alliances are great when known and can help to prevent wars. World War I alliances were secret and therefore no one could predict war on one would actually mean war with many. The Treaty of Versailles after World War I outlawed secret alliances. The alliances following World War II more than likely prevented a World War III during the Cold War.