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."Had Britain and France acted when Hitler sent his troops into the Rhineland, threatened Austria, or even Czechoslovakia -- they could have stopped Hitler at little cost. While it's wise to fear war, it's better to go to war to eliminate a small danger than to allow it to metastasize into a dire threat to your way of life and simply hope against hope that you won't have to deal with it one day. Everybody's not "another Hitler:" Know who's not another Hitler? Pretty much everybody who ever lived except for Adolph Hitler. Maybe you could get away with referring to Stalin or even Pol Pot as "another Hitler," but some off-hand comment in a speech or a policy people disagree with doesn't make a politician "another Hitler." Likewise, a 70 year old guy who gets testy with his congressman at a town hall meeting isn't a "Brownshirt" either. American politics could do with quite a bit less "You're a Nazi" rhetoric being tossed around by both sides.