Although World War II was officially started by Germany invading Poland, there were several events that led up to this war. Some causes include the Treaty of Versailles and the rise of fascism.
The start of World War II has its roots in World War I. After that war ended, the Treaty of Versailles was signed, which ordered Germany's demilitarization and put the onus of the war on Germany's shoulders. Part of the reparations agreed to in the treaty was financial restitution that, along with an unstable government, resulted in a crippled economy. Germany's financial situation worsened when its colonies were taken away, and it had to give up land for Poland.
Meanwhile, fascism was becoming a popular political platform and made its way into the National Socialist German Workers Party or the Nazis. Adolf Hitler was the leader of this party, and made headway into politics until he was appointed Reich Chancellor. From this position, Hitler managed to pass emergency acts that essentially put him in position of ultimate power. Hitler then proceeded to remilitarize Germany and target Jews. Germany then annexed Austria, took over the Sudetenland and seized Czechoslovakia, formed an alliance with fascist Italy, and entered into a pact with Russia. World War II finally broke out in earnest when Germany invaded Poland.