Prussia (Preußen in German), named after the duchy and former Ordenstaat but born of the margravate and electorate of Brandenburg (coincidentally by merging with said Ordenstaat, by then secularized), historically the land of the Baltic Old Prussians, was the descendant of The Teutonic Knights and became the dominant state in Germany (having more than half of Germany's land area and population) by the time it was unified (by Prussia, incidentally) in 1871. Brandenburg, Prussia was the location of the German capital, Berlin. From 1701 until the end of World War I in 1918, it was known as "The Kingdom of Prussia", and post-WWI as "The Free State of Prussia" because "republic" sounded too French. After World War II Prussia ceased to exist because the four Allied powers, in their bid to destroy the Third Reich and its legacy, laid blame on Prussia and its miltarism as a large part of what had gone wrong with Germany and thus dismembered its territory.note After World War II, all of Germany east of the Oder-Neisse linenote , most of which was Prussia, was handed over to Poland, partly to compensate her for her own territorial losses to Ukraine and Russia (the northern part of East Prussia, including its capital, went directly to the Soviets). Germany kept a claim on those areas until 1970, when it signed the Treaties of Moscow and Warsaw. This was again confirmed with the 1992 Treaty of Good Neighbourship, which formally and finally recognized East Prussia, along with Pomerania and Silesia, as part of Poland. The area remains a part of Poland to this day, and almost everyone is happy for it to stay that way. The northern half of East Prussia, however, outlasted the USSR and remains part of Russia as the Kaliningrad Oblast, where there is still talk by some locals (odd, considering said locals are almost all Russians, or perhaps not so odd given that it's currently named after a member of Stalin's inner circle who was personally responsible for some Soviet atrocities, and post-Soviet Russia has reversed most other Stalin-era renamings) to rename the titular city back to Königsberg.note
Earlier on, Prussia's dominance was built in the 18th century on its trademark militarism, which was summarized by Georg Heinrich Berenhort, an adjutant to King Frederick (popularly misattributed to Comte de Mirabeau) in the witticism, "some countries possess armies, but Prussia is an army that happens to possess a country in which it was merely quartered, so to speak." This reached its height in the Seven Years' War, when Prussia essentially stood alone (heavily subsidized by Great Britain) against Austria, Saxony, Russia, France and Sweden. All at once. And not only survived, but kept all of its pre-war territory. This was achieved not merely by skill but also out of sheer luck, as the Russian empress died during the middle of the war and her throne passed to a Prussophile nephew and later to his "grieving" widow who had no interest in pursuing Austria's war. That's why Frederick II is called the Great. Their elan fell during the The French Revolution and its military reforms, which eventually led to Napoléon Bonaparte completely smashing it flat in 1806. In response to Napoleon, the Prussians made a number of reforms inspired by the French Revolution and Napoleon, and once again subsidized and aided by Britain and Russia, they bounced back Back from the Brink. The 1813 Battle of Leipzig, or Battle of the Nations, was the largest military battle in Europe prior to World War II and it was the first time Napoleon at the height of his prowess and skill suffered a major defeat in battle, one from which he never recovered. The Prussians also served as The Cavalry during the Battle of Waterloo, and some Prussians and Germans complain about Britain downplaying their contribution in The Napoleonic Wars (in a manner similar to how people from former Soviet countries complain about America Won World War II).
After the Napoleonic Wars, the Prussians were committed initially to a policy of reactionary stability, opposing German nationalism, mostly because Prussians saw German nationalism as a reduction of Prussian interests and they were especially opposed to the land reform policies and democratization which they saw as the source of the French Revolution and the Napoleonic catastrophe. They were committed to reforms and modernization schemes so long as they put Prussian interest first. This involved the creation of the Customs' Union, and much later, their co-opting of German Unification under Bismarck, after opposing it during the Revolutions of 1848. With the proclamation of the German Empire in 1871 at the Palace of Versailles, Prussia became the part of the new Germany. Many scholars have plausibly argued that Germany was not unified at all, but merely "Prussianized".
After World War I, the Hohenzollern monarchy was overthrown and Germany was forced by the Allies to give up a significant chunk of its eastern territory to the newly-recreated state of Polandnote . This left East Prussia and the old imperial capital Königsberg (plus the neighbouring, short-lived, Free State of Danzig) physically separated from the rest of Germany by a small strip of land known as the Polish Corridor, the existence of which was one of the many many factors that led that Austrian guy with the Chaplin moustache and his friends to start another war. It didn't end well.
Incidentally, while many in the Nazi top leadership had a big nostalgia for the glory days of the Prussian-run Empire and of the Prussian military, most of these men, including Hitler, weren't Prussian at all: they were predominantly southerners (or Austrians like Hitler). Furthermore, while they both supported the idea of a powerful German empire, the Nazis largely had nothing but contempt for the Prussian aristocracy, albeit because they lost the First World War and not out of dislike for their history, culture, or ethos. "Prussiandom" as a concept, as an ideology, and as a national identity had by then become so ephemeral as to be claimed by a very diverse range of individuals with very diverse hopes and fears. Despite their famous and well-deserved authoritarian and anti-democratic reputation, it would not be accurate to assume that all of the Prussian aristocrats were entirely in synch with Nazism. They were in synch with the anti-semitism, the genocide of the Jews, the Roma, the homosexuals, and the Communists, and they were especially on board with the Drang nach Osten and Generalplan Ost and the open season for war crimes. But despite that, a few of them, privately and publicly, disliked the populist and demagogic nature of Nazism, albeit they generally agreed they were preferable to the Social Democrats, Catholics, Socialists and especially the Communists. When the Nazis came to power, seeking to build a more dynamic and up-to-date military institution, they limited much of the aristocratic officer corps' power, rightly seeing them as relics incapable of practicing what they had preached all along.
After the Nazis started enjoying military success, the Prussian officer class had mixed but mostly positive feelings. However a good contingent of them, known to history as Operation Valkyrie, turned on Hitler, perceiving him to be increasingly irrational and privately disagreeing with some of the policies of the genocide (but not all). The famous July 1944 assassination plot which came the closest among many to killing Hitler was led by Prussian aristocrats, and this has allowed them in the post-war era and in recent times to get a Historical Hero Upgrade and a potential correction to the Nazi Nobleman image. The truth of the matter is that the Valkyrie plotters and the aristocrats were mostly war criminals themselves (one of them served with the Einsatzgruppen death squads) and there isn't sufficient evidence to suggest that their assassination attempt was motivated by humanitarian concerns or guilt about the genocide. The little we know of their plan suggests that they intended to force a peace with the Western allies to prolong the illegal war in the East as well as protect and expand their imperialist plans to extend German borders eastwards. Recent documents reveal that this assassination attempt only strengthened Hitler's popularity and support and it hardened the loyalty of the German people to back the Fuhrer to the bitter end (as compared to Mussolini and Vichy France whose peoples rose up spontaneously with the arrival of the Allies).
Prussians historically, right from the time of The Teutonic Knights to Frederick the Great tended to see the East as their Wild West, to which they felt they had their own manifest destiny. King Frederick justified the destruction of the Polish Lithuanian Commonwealth (which he masterminded) by dismissing its citizens as "slovenly Polish trash" no different from the Iroquois of the New World (whom he had read about eagerly) and as such similarly entitled to zero claims of land and citizenship. Ideas of settling Eastern Europe and the Borderlands underpinned Imperial Germany, with many Prussian Junckers refusing to allow land reform policies (that would involve losing their large manorial estates) by insisting on encouraging settlements and expansionist wars in the East. The difference between such plans and the form they took in the Nazizeit is one of degree but not of kind. Some Prussian aristocrats and military men saw their project as benevolent mission civilisatrice and many believed that if they handled their policies properly then the Slavic peoples could be assimilated into their culture as second-class citizens, whereas the Nazis saw them as subhumans (Untermenschen) who could be exterminated without remorse. Indeed the Nazis noted that there were no war crimes in the East because there were no human beings living there, only subhumans. In any case it all came for naught. When the Communists came to power in East Germany they enacted land reform and broke up the Junkers' large manorial estates; these policies remain in effect as a result of the Final Treaty signed by the BDR and USSR with descendants of old Junkers failing to get compensation.
In modern historiography, Prussia remains a controversial subject. A lot of this is because of the Prussians themselves, who in their time described their model and approach to modernization as a Sonderweg ("special path") that was a third way between the plebeian democracy of the West (America, England, France) and the decadent autocracy of the East (the Russian Empire). Post-World War II historians reclaimed the term Sonderweg and applied it as a reason for why Germany, despite its considerable modern and intellectual advancements, became the crucible for the worst crimes of the 20th century and a refutation of all models of linear development from barbarism to civilization. Prussia came to represent everything that was wrong with German historical development: expansionist, militaristic, intolerant, reactionary yet technically competent - in essence, the land which turned Germany into the land of "judges and hangmen" after its centuries as a seat of "poets and philosophers". When it was suggested in 2002 that the name be revived for a proposed merger of Berlin and Brandenburg, one German historian even went so far as to declare that "Prussia poisons us".
Some historians have tried to correct this view. Christopher Clark in Iron Kingdom rejected it as a caricature of Prussia as a "termite-state", arguing in favor of a long tradition of intellectual inquiry, scientific progress, and political tolerance. For instance, during the Catholic-Protestant convulsions that seized Europe in the 16th, 17th and early 18th centuries it even had a reputation for taking in refugees, such as Huguenots from France, Brethren from the Czech-speaking parts of Austria, and Lutherans from the bishopric of Salzburg. The view of Prussia as inherently authoritarian or that its path to modernization was inevitable or inherent in the system does not sit well with contemporary academics inspired by Postmodernism who generally reject such models. More contentious, however, is the question of Prussia's and Imperial Germany's responsibility for World War I. In general those who argue in favor of the Sonderweg agree that Imperial Germany and Prussia were responsible for the outbreak of the war and that they hold primal agency for the outbreak of the conflict. Clark's follow-up book, The Sleepwalkers, largely absolves Imperial Germany from primary responsibility by focusing on collective guilt, and while this book was popular in the Anglosphere (where unfamiliarity with Prussia and Eastern European politics makes them favor collective guilt) it was criticized by historians such as Hans-Ullrich Wehler and Volker Ullrich who noted that Clark rejected the great documentary evidence that showed that Imperial Germany did launch a war of aggression by escalating the Balkan crisis. Likewise, they note that during the Weimar era, many documents and archives were altered and doctored to downplay German guilt, and that the idea of "collective guilt" was encouraged by the Weimar Republic as a PR campaign to cultivate sympathy in the West and to pacify the interests of Junkers who were not fully on board with the democracy. Aside from the "collective guilt" premise, some of those arguing against Prussian (and more generally, German) responsibility for World War I instead assign blame to Russia (arguing that had the Tsar minded his own business, it would've just been a regional conflict between Austria-Hungary and Serbia), a premise which predictably gains more approval whenever there's increased animosity between Russia and the rest of Europe.
Also see Prussian Kings and, for what remains of Prussia, the Brandenburg section in The Sixteen Lands of Deutschland. Compare and contrast Imperial Germany. Also related to Kaiserreich. Wilhelm II, pictured, Kaiser (German Emperor) and King of Prussia from 1888 to 1918, is probably the most famous Prussian in popular imagination, mostly for being the "bad guy" of WWI. Well, he did show up in The Simpsons once.
Another notable Prussian was Otto von Bismarck, exemplar of Germanic Efficiency with a Magnificent Hat to prove it. He is at Number 9 on Germany's list of its top 200, because we all love a Magnificent Bastard. Despite his being deceased this is undoubtedly part of his plan. Just what part, we may never know.