World War I involved one group of countries fighting another.
The Allies of World War I , or Entente Powers , were the countries that opposed the Central Powers in the First World War . The Paris Peace Conference , also known as Versailles Peace Conference , was the meeting of the Allied victors , following the end of World War I to set the peace terms for the defeated Central Powers diplomats from more than 32 countries and nationalities . World War I . The war drew in all the world 's economic great powers , assembled in two opposing alliances : the Allies ( based on the Triple Entente of the Russian Empire , the French Third Republic , and the United Kingdom of Great Britain and Ireland ) versus the Central Powers of Germany and Austria-Hungary . 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 .