Japan entered the First World War in 1914, just a few days after its ally, Great Britain, declared war on Germany. Entry to de conquest by the emerging Asian power would subvert all the geopolitical balance in East Asia and Western Pacific to cause a sudden disappearance of the German presence in the region and the sudden enlargement of the Japanese empire, bringing about suspicious and concern to the country which was the main rival of Japan: the United States. But the USA government not only would feel fear of Japanese expansion but Britain itself, which since 1902 had formed an alliance with Japan in an anti-Russian role in Asia. The British themselves, noting the speed and efficiency of the Japanese forces in its brief fight against Germany in the Far East and how the Japanese demanded the occupied territories (Jiaozhou and the Marianas, Caroline, the Marshall and Palau), feared the new power to become a new dangerous enemy in the short to medium term. For this reason, the two Anglo-Saxon powers, after the USA entry into the war in 1917, ended up forming a common front at the Paris Peace Conference in 1919 (better known as the Conference of Versailles) to ensures their hegemony in post world war and limit the ambitions of Japan. this position of Anglo-Saxon opposition to Japan in key issues like the racial equality proposal for the League of Nations, marked the beginning of a slow but inexorable irreversible process that would break the Anglo-Japanese alliance, would intensify up to its maximum the US-Japanese rivalry and ultimately would separate Japan from de western countries in opposing sides, so setting the stage for the war in the Pacific form 1941 to 1945.