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."
Hitler was not shy about telling people what he intended to do when he reached power. The first volume of Hitler's book Mein Kampf, which included a very rough blueprint of his plans, came out in 1925. Hitler became Chancellor of Germany in 1933 and he swallowed Austria in 1938. Had Europe's leaders simply taken Hitler at his word about what he wanted to do and acted appropriately, he would have been squashed like a bug and humankind would have been spared another world war.
Watch what people do more than what they say: This one may seem to be a bit of a contradiction with the last one, so let me explain.
Surprisingly often, people with bad intentions will tell their followers exactly what they intend to do and then, when confronted by a power that could potentially stop them, whether it be another nation or just the voters who can put them out of office, they will simply lie.
So, if you're not sure what a nation or a leader truly intends, pay more intention to what they do than what they say. It takes a true fool to believe words over actions, but such fools were not in short supply during Hitler's day, nor are they uncommon today.
Diplomacy for its own sake is useless: There was no shortage of diplomacy between Hitler, his victims, and the great powers of the day. The problem was then, as it often is now, that so many people seemed to believe that diplomacy was an end unto itself. Hitler happily met with the representatives from other nations and either bullied them or told them what they wanted to hear. Then, he promptly did whatever he intended to do in the first place. That's why talk alone is meaningless and can even be detrimental if people mistake merely conversing for progress. If you have no carrots and sticks to bring to the table in order to produce the outcome you want, you are wasting your time.
Appeasement is a mistake: When you reward a behavior, it usually occurs more often. So, when a belligerent nation or group benefits from its belligerence, it should surprise no one when it continues to be belligerent. That principle applied to Hitler and it most certainly still applies today.
The mediocrity of political "leaders:" We have a tendency to believe that our political leaders are much better, smarter, and more capable than the average person. In some cases, that's true -- but today, as in Hitler's day, men like Churchill were rare as hens’ teeth while shortsighted, gullible, and foolish "leaders" were the rule. Those who are deeply skeptical of the competence and claims of their political leaders will find that history is almost always on their side.
Be very wary of people building power outside the rule of law: In 1923, Hitler tried to take over Germany with the poorly executed Beer Hall Putsch. Despite the fact that Hitler was convicted of High Treason, a sympathetic judge sentenced him to a mere five years, of which he only served nine months. Additionally, Hitler's own private army, the Brownshirts & the SS, assaulted his enemies, disrupted their political gatherings, and generally paved the way for his rise to power. This is an example of why allowing certain political groups and parties to be "above the law" can be a great threat to democracy.
There are things worse than war: More than 400 years before the rise of Hitler, Machiavelli wrote:
"One should never allow chaos to develop in order to avoid going to war, because one does not avoid a war but instead puts it off to his disadvantage."