Why were leaders like Hitler, Mussolini, and Stalin able to come into power?

At the end of World War I, Europe was in complete chaos. Many were discontent, many were in financial distress, and many grew more angry at what "democracy" had come to. The German people were hit hard by the terms of the Treaty of Versailles. Many were impoverished, and blamed western countries for their demise. Hitler saw an opportunity to speak to the masses. He offered a strong, authoritarian style that many came to crave. Democracy under the Weimar Republic had brought the people to their knees, so Hitler was a solution to the "weakness" and "fragility" of democracy. Mussolini also had a similar experience - he was able to offer Italians a strong, no-nonsense government. It should be noted that Mussolini and Hitler were fascists. They did not support communism - in fact, they said communism and socialism were their sworn enemies!

Stalin and the Soviet Union is more unique. Stalin didn't rise to power like Hitler and Mussolini - he was already running a totalitarian government.