The Convention of Kalisch (28 February 1813) was signed between Russia and Prussia, and committed Prussia to rejoined the war against Napoleon, setting the stage for the War of Liberation of 1813.
After the crushing defeats at Jena and Auerstadt in 1806 and the Russian defeat at Friedland in 1807, King Frederick William III of Prussia had been forced to accept humiliating peace terms. Prussia lost all of her territory in western Germany, while her Polish lands were taken to form the Grand Duchy of Warsaw. In 1812 Prussia was even forced to provide troops for Napoleon's disastrous invasion of Russia.
The Prussia corps, under General Yorck, was allocated to Marshal Macdonald's army, operating to the north of the main French army. Napoleon's retreat from Moscow forced Macdonald to retreat as well. During this retreat Yorck's corps was cut off from the rest of Macdonald's army. Yorck entered into negotiations with a Russian delegation (including Clausewitz), and on 30 December 1812 signed to the Convention of Tauroggen. His corps became neutral, and was allowed to retreat into East Prussia.
This caused a problem for King Frederick William III. Prussia was still largely occupied by the French, and there were even French troops in Berlin. At first he rejected the convention, but Yorck's action was very popular in Prussia. Early in 1813 negotiations began between Prussia and Russia.
On 28 February the Prussian chancellor Karl August Fürst von Hardenberg and Marshal Kutuzov signed the Convention of Kalisch (Kalisch was then in the Duchy of Warsaw, and is now Kalisz, Poland).
The treaty had twelve public and two secret articles, although Article XI also stated that the entire treaty would be kept secret for two months. Articles I and II ended the war between Prussia and Russia and replaced it with an alliance between the two countries. Article X saw Prussia agree to provide provisions for any Russian troops based in Prussia. Prussia and Russia both agreed to provide 150,000 troops for the war, and not to negotiate unilaterally with Napoleon.
The secret articles covered territorial changes. Russia agreed that Prussia should regain all of the German territory lost in 1807 at the Treaty of Tilsit. The Russians were less generous in Poland - here Prussia was only to keep the territories it had gained in the First Partition of Poland of 1772 and not the lands taken in the Second or Third Partitions (with the exception of a narrow strip of land on the route to Prussian Silesia). The rest of Poland would go to Russia.
The Russians broke the secrecy clause and revealed the existence of the treaty one month early. However by that point the Prussians were ready to enter the war. The French had already evacuated Berlin (4 March), and on 17 March 1813 Prussia declared war on France, publicly entering the War of the Sixth Coalition.