What three major themes of European history have defined the era of 1804–1939?

It is hard to find themes for this period of European history, as it deals with different time periods. One theme would have to be that European nations formed various alliances. Nations banded together to fight against the armies of Napoleon during the Napoleonic Wars, Czarist Russia during the Crimean War, and the armies of Hitler and Mussolini during World War II. World War I was partially started by an alliance system fueled by the rise of German nationalism under Kaiser Wilhelm II. Britain's role in the alliance system was always to make sure that no one power gained too much control over the continent. Britain went to war with Russia in the Crimean War to ensure that Russia did not gain too much strength in the Eastern Mediterranean. Britain also sided with the Entente during World War I because it feared that a strong Germany would ultimately be a threat to Britain's interests in Europe.
Another theme for this period is European nationalistic movements. These were especially pronounced in the period immediately after World War I as the Baltic countries, Czechoslovakia, Yugoslavia, and Poland sought their independence from the Russian and Austro-Hungarian empires. Germany united under Prussian leadership in 1871, and the modern nation of Italy formed at around the same time period. Greece became its own nation in the early part of the nineteenth century, and Bulgaria fought and won a successful war against the crumbling Ottoman Empire in the latter part of the nineteenth century, during the Balkan Wars. This nationalism would also help fuel both World War I and World War II, as World War I was ignited by a Serb nationalist who assassinated the Austrian archduke in Sarajevo. Hitler's stated goal was to unite German-speaking people who were separated from the empire after the Versailles Treaty.
A final theme for this period could be the drive for colonies. While this started before the period in question, the major powers of Europe still coveted colonies during this time period. Russia maintained Alaska until 1867, when the United States purchased it. Italy and Germany had colonies in Africa, with Germany controlling some islands in the Pacific. The British Empire still stretched all over the world and was maintained by the British Navy, the strongest navy in the world until 1918. France controlled lands in Africa and Asia. Even Belgium controlled a major portion of central Africa. The great powers of Europe considered colonies a source of national pride, and these would also help to fuel the fires that led to World War I.
While there are other themes during this period, such as intermarriage among royal families, the relative period of peace known as the Pax Britannica that took place between the Crimean War and World War I, and the rise of European industrial output during this time period, I think that examining the foreign policies and diplomatic histories of this time period is its most interesting facet.