The key causes of WWII were the rise of dictators like Hitler in Germany, Mussolini in Italy, and Hirohito in Japan. Germany's economy was ruined after harsh treatment following WWI, including huge reparation payments. Countries were also aggressively expanding their borders in the 1930s, which the weak League of Nations struggled to address. Finally, Germany attacked Poland in 1939 and Japan bombed Pearl Harbor in 1941, pulling the world's major powers into a second global war within two decades.