What caused world war 2?

World War 2 was caused mainly because the nations tensions, issues that where unresolved, resentments from World War 1, and some effects of the great Depression. World War 2 involved many of the world nations, and over 100 million military troops. This was the most wide spread war in history. 70 million people and more were killed, making it the deadliest conflict in human history. The war began on September 1, 1939, when the Germans started to invade Poland. soviet union invaded Poland from the East side. Other countries were at war already before Poland was invaded by the Soviet union. about 16 days later Germans invaded the Soviet union. In 1945 the war ended the Soviet union and the United states then had the cold war which lasted for 46 years it ended on many different dates because of all the different wars happening. Here is a helpful source that you could use to learn much more about World War 2.