The Convention of Schönbrünn (15 December 1805) was an alliance between Prussia and France forced on the Prussians in the aftermath of Napoleon's great victory at Austerlitz.
On 3 November 1805 the Prussians had signed the Treaty of Potsdam, agreeing to enter the Third Coalition if Napoleon didn't agree to peace within four weeks of the departure of an envoy from Berlin. This treaty came to nothing after the battle of Austerlitz, and instead the Prussian envoy, Christian Haugwitz, was forced to negotiate a treaty of friendship.
Prussia was forced to agree to a formal alliance against Britain. She was also forced to surrender three outlying parts of the kingdom. Cleves, on the lower Rhine was given to Murat, who became Grand Duke of Berg and Cleves. Ansbach, an isolated Prussian enclave in Bavaria, had been crossed by Bernadotte's corps early in the War of the Third Coalition, and was now given to Bavaria. Neuchâtel, on the western edge of Switzerland, was given to Napoleon's hard working chief of staff Berthier. In return Prussia was offered Hanover.
Haugwitz was forced to sign the convention on 15 December 1805. It was then sent to Prussia to be ratified. King Frederick William III of Prussia attempted to delay this, but was eventually left with no choice, and ratified the treaty on 24 February 1806. Prussia was left diplomatically isolated, and Napoleon now only faced Britain and Russia. Peace negotiations with Russia came quite close to success, but Napoleon's attitude had pushed Prussia towards the war camp. The peace party's cause wasn't helped when Napoleon offered to return Hanover to Britain in return for peace. Much to Napoleon's surprise later in 1806 Prussia joined the Fourth Coalition, and war was renewed.