Was appeasement the main reason for the start of war in 1939?
Appeasement helped to determine when the war would start, but it did not cause the war. The cause of the war was Hitler's desire to get revenge for the Treaty of Versailles and to create the empire that he felt Germany deserved.
Without appeasement, it is possible that the war would have started earlier. The Allies could have gone to war earlier than they did, perhaps when Hitler pushed for the Sudetenland. However, the actual start of the war was caused by the end of appeasement. Hitler's aggression extended to an invasion of Poland and the Allies stopped appeasing him and declared war.
So, the true cause of the war was Hitler's aggression. Appeasement might have caused the war to start in 1939 rather than in 1938, but it did not cause the war.