Do you think United States isolationism encourage or discouraged future acts of aggression by dictators?

Neither. Isolationism is the same as the friend that wants to stay out of an argument. However, US was secretly sending supplies to Britain during the first world war, and resulting in their ship being sunk. That is one reason they joined that war. In the second world war they played the isolation card and were forced to go to war after the occurrence of Pearl Harbor.