Causes:

Italy becomes more a aggressive country. Mussolini and invades Ethiopia and is easier becuase of their more advanced milliatry. .

The United States cuts off supplies to Japan, which makes Japan angry. This would further lead the attack on Pearl Harbor .

Japan attacks Pearl Harbor and brings the U.S inot the war, which was a big mistake becuase the U.S was stronger.

Hitler creates his Nazi party and believes strongly in the white Aryan race be superior and that Jews were a inferior race. 

Hitler broke the Versailles treaty and created a much bigger army than the one before.

Effects:

A lot of important people were killed during this war such as doctors who could have found cures for many diseases if they were still alive.

Jewish people were persecuted during this war and millions of them died in concentration camps becuase Hitler had placed them there.

Many countries had a hard time rebuilding after the war and did not have the funds to to do so. Also some never got the chance to becuase of the Cold War which started right after.

Many of the countries that were involved in the fighting need supplies and would get them from the places near by and aslo take many natural resources.

The Atomic bomb was dropped which help lead to the end of the war. It also killed thousands of Japanese people, which led to the discussion weather or not we should have dropped the bomb.