The policy of appeasement was to blame for Hitler's early successes because it allowed him to build power while simultaneously preventing France and Britain from doing the same.
Hitler was able to build his power because the Allies wouldn't oppose him. They simply allowed him to break the Treaty of Versailles. They allowed him to take the Sudetenland and Czechoslovakia. This made Hitler strong in his own mind and in the eyes of his people.
At the same time, appeasement made the Allies think they could avoid war. They didn't really get ready for the war and were totally unprepared to deal with the Germans and their new ways of war. This allowed Hitler to win easily in 1939, 1940 and 1941.