World War I included the Allies having conflict with the Central Powers.
The Allies of World War I , or Entente Powers , were the countries that opposed the Central Powers in the First World War . World War I . It was one of the deadliest conflicts in history , and paved the way for major political changes , including revolutions in many of the nations involved and to WWII twenty-one years later . These alliances were reorganised and expanded as more nations entered the war : Italy , Japan and the United States joined the Allies , while the Ottoman Empire and Bulgaria joined the Central Powers .