Answer:
A number of aspects all contributed to the outbreak of World War II.
For one, after the closing of World War I, Germany was treated extremely unfairly by the world in the Treaty of Versailles. Their country was essentially pushed to the ground and treated like dirt. Completely stripped of pride and their former Imperial glory. Germans, as you can guess...were not happy about this.
Adolf Hitler came along with the National Socialist Party (Nazi Party) and spoke of redeeming Germany to its former military and economic strength. You have to understand, the Germans were a very proud people, and were angered beyond repair by their treatment. They were also having economic struggles, which Hitler promised he would repair with strong and reassuring speeches. He told them of the glory and honor he would bring back to them, and how he could bring Germany back to its former strength and beyond!
The people fell in love with his strong and charismatic speeches, and so in 1933, he was elected Chancellor of Germany. From 1933-1939 Hitler planned to invade Poland and other ex-German territories. He built up a massive Army of nearly 6 million volunteers, trained, and disciplined them into an elite, unstoppable war-machine. Then when the chance came along, he invaded Poland on September 1st, 1939. Britain and France then declared war on Germany, as Poland was their ally. This officially marked the beginning of the Second World War.