Der Anschluss

March 12, 1938 is probably an insignificant date for most Americans. Even Americans who were alive during that time probably couldn't tell you the significance. If you go to Austria, however, you'll see signs on this day that say things like never forget. I suppose they hope the coming generations will never forget this was the day Hitler was allowed to start his take over.
Der Anschluss literally means affiliation or alignment. It was a perfect word for Hitler's propaganda, trying to reinforce the idea that Germany and Austria should be unified. What really occurred was political corruption leading to the annexation of Austria to Germany. People blame Austria for "allowing" this to happen and for being happy with the Anschluss. In actuality, they were happy that it wasn't a bloody takeover, as it very easily could have been. Let's face it, if Hitler wanted Austria he would have it by whatever means necessary.
Retrospectively, the allies are to blame for the Anschluss. Or, at least, for saying nothing and doing nothing in reaction to it. Everyone just turned a blind eye, or merely verbally chastised, giving Hitler the motivation and ability to continue his take over of Europe. Sure, a few people were critical, but they could always be silenced by the masses.
Der Anschluss was the beginning of Hitler's rise, so it's important to remember. Read more about it here.

"Those who cannot remember the past are condemned to repeat it." -George Santayana