Although the immediate cause of WWII was the German Invasion of Poland, there were many factors leading up to this point. 

One of the key causes of WWII was the German dissatisfaction about the end of WW1.  After the Armistice was signed which stopped the conflict, the Treaty of Versailles took six months to negotiate.  The treaty was negotiated internally by France, England, United States, and Italy.  Germany was required to sign or the war would recommence.  The treaty covered many topics and requirements

Germany was required to turn over approximately 65,000 km2 to various other countries.  This amounted to over 7 million people.  As well, the total coal output of Saar coal mines was to be turned over to France for a period of 15 years.

All colonies and territories of Germany outside of Europe was turned over to other countries.

 The German Military was heavily restricted.  The Army was allowed only 7 infantry and 3 calvary division -100,000 men.  No tanks, armoured cars, or air force was permitted. The Navy was restricted 10 15,000 sailors and to a limited number of ships.  All defensive fortification on or 15 miles to the east of the Rhine were to be demolished and could not be rebuilt.  Germany was banned from buying or selling weapons of war.

The harshest section was article 231 which required Germany to pay to the Allied Powers, war reparations. The amount and form of these payments would be dictated to Germany.  It required an initial payment of an amount equal to 5 billion US Dollars immediately and in 1921 capped the reparations at 56.5 Billion US Dollars. Please note that those are the amounts at that time and not corrected to today’s inflated amounts.

Lastly, the Rhineland would be occupied by Allied troops for a period of 15 years.  In the event that there was no offensive action by the Germans; a staged withdrawal over another 15 years would take place.

The treaty was denounced throughout Germany and by many on the Allied side as being too harsh and punitive. The treaty humiliated the German people and destroyed the already weak German economy.  In the end, only 33 billion US dollars of reparations were paid back.

The German government and military went to great lengths to get around the treaty.  German soldiers trained on tanks and air craft in the Soviet Union and Sweden. German armament companies moved to the Netherlands, Sweden, and Switzerland to get around the treaty.   

In December of 1931, the German Government authorized a large budget to start rebuilding its military infrastructure including tanks and aircraft.  In 1932, the army grew past the maximum number of 100,000 men to almost 150,000.  In 1932, Adolf Hitler and the Nazi party took power and effectively renounced the Treaty.