<P> Others follow the British historian A.J.P. Taylor, who held that the Sino - Japanese War and war in Europe and its colonies occurred simultaneously and the two wars merged in 1941 . This article uses the conventional dating . Other starting dates sometimes used for World War II include the Italian invasion of Abyssinia on 3 October 1935 . The British historian Antony Beevor views the beginning of World War II as the Battles of Khalkhin Gol fought between Japan and the forces of Mongolia and the Soviet Union from May to September 1939 . </P> <P> The exact date of the war's end is also not universally agreed upon . It was generally accepted at the time that the war ended with the armistice of 14 August 1945 (V-J Day), rather than the formal surrender of Japan (2 September 1945). A peace treaty with Japan was signed in 1951 . A treaty regarding Germany's future allowed the reunification of East and West Germany to take place in 1990 and resolved other post-World War II issues . </P> <P> World War I had radically altered the political European map, with the defeat of the Central Powers--including Austria - Hungary, Germany, Bulgaria and the Ottoman Empire--and the 1917 Bolshevik seizure of power in Russia, which eventually led to the founding of the Soviet Union . Meanwhile, the victorious Allies of World War I, such as France, Belgium, Italy, Romania and Greece gained territory, and new nation - states were created out of the collapse of Austria - Hungary and the Ottoman and Russian Empires . </P> <P> To prevent a future world war, the League of Nations was created during the 1919 Paris Peace Conference . The organisation's primary goals were to prevent armed conflict through collective security, military and naval disarmament, and settling international disputes through peaceful negotiations and arbitration . </P>

What finally brought the united states fully into world war ii