The Treaty of Potsdam (3 November 1805) was an agreement between Prussia and Russia in which the Prussians agreed to join the Third Coalition if Napoleon didn't agree to peace terms.
The Prussians had stayed out of the Third Coalition, but by early November 1805 Napoleon seemed to be in trouble. He had captured Vienna, but was now apparently isolated in Austria, far from his bases and facing an ever-increasing Austro-Russian army. Tsar Alexander visited Berlin on his way to the front, visiting King Frederick William III of Prussia.
The Prussian king was finally persuaded to join the Third Coalition, but on terms. He would provide 180,000 men to the coalition army but only if Napoleon refused to make peace within four weeks of the departure of a Prussian envoy from Berlin.
Christian de Haugwitz, the envoy, reached Napoleon's headquarters in late November. Napoleon had a good idea of his real task, and made sure that Haugwitz never had a chance to issue his demands. On 2 December 1805 Napoleon won his dramatic victory at Austerlitz, knocking the Austrians out of the war. This effectively ended the Third Coalition, and the Prussian envoy quickly abandoned his mission, cancelling the Treaty of Potsdam. Instead the Prussians were soon forced to sign an alliance with France (Convention of Schönbrünn), surrendering territory and agreeing to enter an alliance against Britain in return for the promise of Hanover.