The direct cause of WWI was the assassination of Archduke Franz Ferdinand at Sarajevo on 28 June 1914. However historians feel that a number of factors contributed to the rivalry between the Great powers that allowed war on such a wide-scale to break out.
Among the causes of World War II were Italian fascism in the 1920s, Japanese militarism and invasions of China in the 1930s, and especially the political takeover in 1933 of Germany by Hitler and his Nazi Party and its aggressive foreign policy.
Outbreak of World War II (1939) ... On September 1, 1939, Hitler invaded Poland from the west; two days later, France and Britain declared war on Germany, beginning World War II. On September 17, Soviet troops invaded Poland from the east.