As an adult, I became a conservative who constantly searches history to find out how on earth we got into this fix, and it wasn't long before I discovered what my teachers had never told me, and probably didn't know: That it was the Great War that gave birth to the modern world, and destroyed the old one.
This fascinating essay discusses how the First World War caused the beginning of the end of monarchy, and allowed the rise of the Nazi and Soviet regimes:
The Tragic Death of the Habsburg Empire
* Eventually, however, the German Empire was re-created as Nazi Germany, and the Russian Empire was re-created as the Soviet Union. By the late 1930s, each was moving decisively to fill the vast power vacuum that Mitteleuropa had become. The first successor empire to that the Habsburgs became that of the Nazis. After the Second World War, a major part of which was fought over Mitteleuropa, the Soviets became the second. It is fair to say that, by 1950, there were almost no nationalists left in any of the successor states who concluded that, all things considered, they preferred the rule of either the Nazis or the Soviets to that of the Habsburgs. Since the national independence of the successor states proved to be an illusion, if not a delusion, their peoples would have been wiser to stay with the empire with which they first began. As it was, there was a certain amount of dark and cosmic justice in their having to pay the consequences for their hubris and their foolishness in bringing about the Habsburg Empire’s dissolution.
* The most interesting counterfactual history, however, would have unfolded in the arena of international politics. Let us stipulate that Nazi Germany and the Soviet Union would have developed largely as they actually did, and each would seek to expand into the Central Europe that lay between them. However, if the Habsburg Empire had survived, they would have confronted a Catholic, conservative Great Power there, one which would have good reasons to oppose each of them, not only because of power-politics calculations, but also because of ideological and theological convictions.
* In the actual history, however, the absence of the Habsburg Empire opened the doors to the Nazi conquests of Austria, Czechoslovakia, Poland in 1938–39; to German domination or invasion of Hungary, Romania, and Yugoslavia in 1940–44; to the invasion of the Soviet Union in 1941; and to the Holocaust in 1941–45. And finally, after the Soviet Army ultimately prevailed over the German Wehrmacht in the longest and most deadly military campaign in history, it surged into virtually all of Mitteleuropa, bringing with it rape and pillage, death and destruction, and Communist revolution. Stalin established a brutal second successor empire to the Habsburgs, tearing Mitteleuropa away from its dense ties with the West and converting and reducing it into being merely Eastern Europe.