The main cause of World War II was the rise of the Nazi Party in Germany and its subsequent invasion of other countries. The causes can be linked back to World War I. The main effects of WWII include the Cold War, occupation of territories and the widespread destruction in Western Europe.
After World War I, Germany fell into a depression, which left the country ripe and ready for a new government regime to come in. The Nazi Party filled that gap and brought the country out of its depression. To do this, they put blame on marginalized groups, such as Jewish and gay citizens. This was the beginning of the Holocaust, and the Nazi Party eventually pushed out of the country into surrounding nations, officially starting the war. There was also worldwide depression, which gave rise to dictators in both Japan and Italy, the two other main antagonists of the war.
After the war, the Allies split Germany into different territories for occupation. Other Axis countries were occupied as well, but the German split is the best known. The split between Western Germany, which was occupied by Western European countries and the United States, and Eastern Germany, occupied by Russia, was enforced by the Berlin Wall. This split was also the beginning of the Cold War, during which the United States and Russia rushed to develop weapons and space technology.
Western Europe was devastated because of the heavy bombings throughout the war, and this led to a long period of rebuilding. The United States played a large role in helping these countries rebuild.