How did the Treaty of Kalisz help begin Prussia’s fight against Napoleon?

The Treaty of Kalisz, signed on February 28, 1813, served as the formal signal for Prussia’s entry into the war against Napoleon.1

Following the collapse of Napoleon’s invasion of Russia, Karl, Reichsfreiherr vom und zum Stein—acting as a political adviser to Tsar Alexander I—negotiated the alliance to secure a formal military partnership between Russia and Prussia. The treaty was critical for the following reasons:21

• Formal Alliance: It established a binding agreement between Alexander I and King Frederick William III, ending Prussia's forced alliance with France.23
• War Declaration: It paved the way for Prussia to officially declare war on Napoleon on March 16, 1813.2
• Strategic Commitment: Frederick William III pledged not to make peace until Prussia was restored to its pre-Tilsit size and population, providing a clear objective for the German War of Liberation.2
• Military Coordination: The alliance was essential to Alexander’s plans, as it combined Prussian troops with Russian forces to challenge French hegemony in central Europe.2