Germany's Defeat in WWI and the Treaty of Versailles which caused Germany to become unstable.
What did each side do to upset the other?
Axis vs Allies. Treaty of Versailles made it so Germany could basically not protect themselves and for them to have an economic collapse which made it possible for Hitler to come to power. Hitler formed Ties with Rome, Berlin and later Japan and other smaller powers joined the axis side. Germany Invaded Poland on Sept. 3, 1939. On that same day France and the UK declared war on Germany. British Troops were sent to France.
Was the conflict inevitable?
Yes, After the Treaty of Versailles was signed there was going to be some type of conflict in order to let Germany have back their land and power. The presence of three respective, powerful states with leaders who would use any means necessary to achieve their dangerous goals, as well as the lack, of any effective check on the actions of those states and their leaders.
Is there anyone to blame for starting the conflict
Some could argue that the Allies side caused it by having The Treaty of Versailles but others would say that they Axis side caused it due to their full scale war set off of invading Poland on Sept 3.
What event set off the conflict?
The actual set off for the event would be the signing of The Treaty of Versailles but the actual start of the World War was Germany Invading Poland.