In reference to Nazi foreign policy, was appeasement the main reason for the outbreak of war in 1939?
There were far greater forces behind the Nazi foreign policy that led to wars of aggression than simply the fact that foreign powers did not aggressively try to stop them. So appeasement is certainly not the main cause for the outbreak of war, though it may have contributed heavily to the timing of the war's start in 1939.
The thing that appeasement did was to magnify the imbalance in the equation that measured the cost of war. Given that the Nazis had seen time and again that the foreign powers were not willing to come to blows over the treaties they'd signed, they knew that they could use war, in particular blitzkrieg, to gain their objectives quickly and without great cost militarily or financially so they felt they had an advantage that way.