A brief summary of the difference between Nazi Germany and the Al-Assad regime...
Hitler first spent nearly a decade turning Germany into an IMMENSE police and war state. He then began waging war on minorities and other unfortunates, in his effort to cleanse the German people and thus enact the grand final solution. This may seem comparable to the Al-Assad regime but even if we are willing to simplify this, really heavily oversimplify, the scales are so different. Massively, cartoonishly different.
But here's where the Third Reich truly diverges from the Syrian state. The Nazis declared themselves the supreme race, superior to all other nations, and literally invaded Europe, smashing nations to bits along the way. It was an international war against a clear conquerer of nations (Hitler) and would eventually explode into a bipolar fight between a large number of allied and axis powers.
The United States did not enter the war the moment Hitler began exterminating Jews. We didn't start a war before that when the Ghettos were constructed. In fact, it would be many years between the first mass killings and our entrance into the European theatre.
If you cannot understand the difference between, arguably, the largest and most brutal war in modern human history and a Syrian civil war that has gone to the dogs, you need to go back to school.