Hitler did not rise out of a vacuum: Many people assume that another Hitler can rise up in any nation, but that's not necessarily so. Hitler's rise in Germany was not a forgone conclusion in Germany, but there were a number of conditions that made that country especially susceptible to it.
The Germans were a warlike people who were used to capitulating to authority and they had a long, rich philosophical bent towards hatred of the Jews and racial superiority. They also had minimal experience with democracy, a terrible economic crisis, the Versailles Treaty, which was an almost universally despised boot placed upon the nation's neck, and an independent military that played a powerful role in political affairs. Some nations, the United States included, have a character that simply precludes their being run by a "Hitler," no matter what the intentions of a leader may be.
All it takes for evil to win is for good men to do nothing: Many people are aware that Britain, France, Russia, and the other powers of Europe had the opportunity to stop Hitler, but the truth is that the German people had countless chances to do so as well.
When Hitler became Chancellor, the Nazi Party had never captured more than 37% of the vote and much of the rest of Germany considered them to be frightening gutter trash. In other words, 63% of the country didn't support Hitler and strongly suspected he was a dangerous man; yet they made no serious effort to stop him. On numerous occasions, Germany's political and military establishment were given excuses and openings that could have been used to bring Hitler down before he came to power and brought the nation fully under his control. Time and time again, people who knew better simply stayed quiet or decided to step aside rather than take a stand. The price Germany and the rest of the world paid for their failure to act is incalculable.
Take even non-reasonable claims seriously: Margaret Thatcher once said,
"It is one of the great weaknesses of reasonable men and women that they imagine that projects which fly in the face of commonsense are not serious or being seriously undertaken."