I am sure that there is hardly a person in existence that has not seen at least a couple of parodies of the scene from this film where Hitler dissolves into a shouting maniac, so most people are aware that it exists, but not that many have seen it. In the film we see the combination of Hitler's descent into delusion combined with his intent to damage the German people in retribution for their having "failed" him.
This was an outstanding historical movie under any circumstances, but in the last couple of days it has taken on a frightening relevance to Americans. If you have never seen this film, or even if you have not seen it in a long time, I urgently recommend that you watch it. It is widely available online, in some cases for free. The parallels between Hitler's decline and the way Trump is now acting are deeply disturbing.
In a spirit of full revelation, I worked on a movie directed by Oliver Hirschbiegel, who was the director of Untergang. I formed a pretty high impression of him.