The Convention of Tauroggen (30 December 1812) was an agreement that made General Yorck's Russian corps neutral, marking the start of a break between Prussia and France (War of Liberation).
After the disasters of 1806-1807 Prussia had been forced to become an ally of France, but this was always unpopular in the country. Napoleon himself was torn between a desire to prevent the Prussian army from reviving and a need for Prussian troops, and in 1812 he ordered Prussia to raise an Auxiliary Corps, to take part in the invasion of Russia.
The Prussian Corps was allocated to Marshal Macdonald's force, which operated on the left flank of the French invasion, in the Baltic. At first it was commanded by General Grawert, but in August General Yorck took over after Grawert fell ill (the official change wasn't made until 12 October). Macdonald's main task was the siege of Riga, which successfully held out for the entire war.
As the defeated Grande Armée retreated from Moscow, Macdonald's position in the Baltic became increasingly dangerous. On 18 December he was ordered to lift the siege of Riga, and his entire force began to retreat back towards East Prussia.
During this retreat Yorck's corps was cut off by Russian troops, who got between the Prussians and Macdonald on 25 December. Yorck immediately entered into negotiations with the Russians, although the key figures in the Russian delegation were all Prussians – Generals Diebitsch, Clausewitz and Dohna, Prussian exiles who couldn't tolerate the alliance with France.
These negotiations were successful. On 30 December the two sides signed the Convention of Tauroggen (meeting in a mill at Poscherun, just inside Russia). Under the terms of the Convention Yorck's corps became neutral. It would no longer fight the Russians, and were allowed to retire into northern East Prussia, where they would wait for orders from King Frederick William III.
The Convention of Tauroggen had wide ranging results. Most immediately it forced Macdonald to evacuate Königsberg, the capital of East Prussia, on 4 January 1813. The Russians entered the city on the same day.
More significant was the impact in Prussia. On 6 February 1813 the Estates of East Prussia declared war on Napoleon. This triggered a series of anti-French revolts across Prussia. King Frederick William III was placed in a very difficult position, as there were still French troops in Berlin. At first he condemned Yorck's actions and on 6 January even ordered Kleist to arrest him, but public opinion soon forced the King to change his attitude. On 28 February Prussia signed the Convention of Kalisch, a secret treaty in which she agreed to join the war against Napoleon. On 11 March Yorck was exonerated for signing the Convention, and on 17 March Prussia officially declared war on France.