Throughout history, whenever foreign adversaries have attacked Russia, winter has always been the Russians' greatest ally. The Russian winter was as decisive a force during Hitler's Russian campaign during World War II as it was for Napoleon. There are several military parallels between World War II and the Napoleonic Wars. Both Napoleon and Hitler allied with the Russians for a while, only to have the Russians turn against them. Both sent their armies into Russia, and in both cases, those armies met with devastation.
Yet the Russian winter was not the only reason for Napoleon's defeat: the army he rallied in 1813 may have been 300,000 strong, but it was much weaker than the previous army Napoleon had commanded. Instead of war-hardened veterans, this new army consisted of inexperienced soldiers, many in their teens. The German states of the Confederation of the Rhine left Napoleon's orbit and fought against him. Austria, Prussia and Russia now had highly nationalist (and thus highly dedicated) armies (like the French armies that had been defeating them for the last decade) rather than primarily mercenary armies. In terms of the number of troops used, Leipzig was of unprecedented size.
Following Napoleon's fall, considerable debate arose among the anti-Napoleonic alliance over what should be done with Napoleon, and who should rule France. Czar Alexander I, who by now had a personal grudge against Napoleon, wanted him off the throne. The conniving Metternich, on the other hand, thought a weakened Napoleon (still married to Marie Louise, or course) might serve Austria's interests. After considerable wrangling, the old Bourbon dynasty was restored. However, the legal equality under the Napoleonic Code remained, and thus the French Revolution's achievements survived in part, even with a Bourbon back on the throne.
Europe, too, now needed to be reorganized. At the Congress of Vienna (1814- 1815), all the European powers were represented by their various foreign ministers, including Metternich, Talleyrand, and Robert Stewart Castlereagh (of Britain). This group tried to create a balanced Europe in which no one nation would be too powerful, and peace would prevail. One of the biggest problems in the negotiations was the question of what to do with Poland, which Alexander so desperately wanted. In the end, Britain and Austria aligned with Talleyrand's France to stop the Russians from annexing Poland.