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In the first year of 20th century, Europe wanted expansion and American and Japanese had economic interests en commun, and this is why created contrast between the nations.
In the Europe two opposing alliances formed: The Triple Entence (Britan, France and Russia) and The Triple Alliance (Germany, Austria and Italy). -
In 1914, the assassination of the heir to the Austrian-Hamburg throne Franz Ferdinand had caused the first world war. Austria declared war on Serbia and Russia and France went to Srbia's aid.
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In 1915 Italy entered in the war against Austria, with a Pact of London: the king of Italy, Vittorio Emanuele III signed a treaty that committed Italy to entered the war against Austria in exchange for extensive territorial concessions.
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In 1917, the Russian army retreated as a consequence of the Bolshevik Revolution and the United States entered the war in support of the Allied Forces, after Germany had launched submarine attacks on American ships.
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In 1919, the victorius nations took part in a post-war summit in Versailles, named the Peace of Versailles; to establish the new structure of Europe after the defeat of the central empires.