After an agreement between the two countries, where the British offered to pay 1,250,000 pounds a year for every 100,000 troops Russia put up, they agreed to join the fighting on 11 April 1805.

[15] The Treaty of Potsdam was signed on 3 November 1805 between Alexander I of the Russian Empire and Frederick William III of Prussia.

[18][16] In the treaty, Prussia agreed to mediate negotiations between the French Empire and Russia in exchange for British subsidies.

If the attempt at mediation failed, Prussia agreed to raise an 80,000-man army and join the war on the side of the Third Coalition.

[16][19] Historian Michael Broers argues that Fredrick William was reluctant to join the coalition, as was relatively pro-French Christian von Haugwitz, the foreign minister of Prussia.

[20] While Haugwitz was travelling, Napoleon decisively won the Battle of Austerlitz against Austria and Russia on 2 December 1805, effectively ending the war,[15] and 'destroying' the treaty.