Does anyone know how Prussia managed to come off so well at the Congress of Vienna? I know the Hohenzollerns had limited holdings around the Rhine since the 1550's, but handing them Westphalia, the Rhineland, and so much of Saxony seems like a bit of a leap. Was it simply compensation for carving off Congress Poland, and nobody wanting to recreate hundreds or thousands of German principalities again?
Prussia was one of the founding members of the alliance that eventually defeated Napoleon, along with Russia and Sweden. (The British, despite being at war with Napoleon already, didn't really enter the alliance until well after the Prussians and Russians had begun fighting in Saxony together.) As part of the Russians' campaign to a) get Prussia back in their corner and b) use Prussian manpower and bases to push Napoleon out of Central Europe, Aleksandr offered support for several key Prussian foreign policy goals. Prussia's chief objective was a restoration of the borders of 1806 - before Napoleon provoked the war with them - at which point they controlled Hannover and most of the parts of Poland that were worth having (Warsaw, the Wielkopolska, etc.), along with several chunks of the Rhineland and Franconia. Since these borders were probably unattainable in toto - Britain would demand Hannover back, and the Russians were interested in Prussian Poland - the Prussians were willing to settle for equivalent territory.
Since Prussia was such a huge contributor to the defeat of Napoleon - thought it must be said that without
any of the 'big four', the alliance probably would have failed - they ended up, more or less, getting their wish. The 'more or less' is a rather big departure from the actual plan, though. Russia had in effect promised the Prussians total control of Saxony in exchange for their share of Poland. This was supposed to be a fairly easy pill for the Austrians and British to swallow, since the Saxon king had been one of Napoleon's staunchest allies and supposedly deserved to be punished. It turned out that Saxony's total destruction was not acceptable to those powers, which launched a serious crisis in the winter of 1814-15 (inventively enough called "the Saxon-Polish crisis").
The end result was that Russia won - gaining Prussian Poland in the face of the opposition of the British, Austrians, and Talleyrand - but then promptly forced Prussia to settle for "only" about half of Saxony. In exchange, the Prussians would get the remainder of the Rhineland, partially to give them a stake in defending western Germany from the French, partially in order to keep it out of the hands of Willem I (who wanted to resurrect some sort of Burgundian state), and partially because the Prussians had owned a pretty significant chunk of it before the revolutionary wars
anyway. The Rhineland pretty clearly could not go back to the way it had been in 1792 - something like three hundred independent polities controlled it, which had been swept away by Napoleon - and it could not go back to the way it had been under Napoleon, when it had been ruled by France and the Bonapartist sock-puppet, Westphalia. Prussia was the most acceptable solution.