Up until the Battle of Borodino, Napoleon and his Grand Armee looked invincible. However, his invasion of Russia proved disastrous, and at Borodino, on September 7, 1812, he lost up to 35,000 men and almost 50 generals. Although he captured Moscow after that, the bitter Russian winter obliterated his army. When it was all said and done, the Grand Armee had lost almost 400,000 men at the end of the campaign and Napoleon had to return to France and lick his wounds.
Despite this setback, however, Napoleon was still a grave threat to Europe and had yet to be decisively defeated in battle. Even so, the armies of France were on the retreat across Europe, so several of Europe’s biggest powers combined to form the Sixth Coalition. By working together, the Coalition was able to muster up an enormous amount of resources, and ultimately, it was able to use sheer weight of numbers to beat Napoleon. The French Emperor quickly rebuilt his army after the Russian fiasco and was confident he could defeat the might of the Coalition. He was proven wrong, and at the Battle of Leipzig, his first major loss was the start of his downfall.
When Napoleon lost most of his army in Russia, some may have thought there was a possibility it would mark the end of his attempts at conquest. They were dead wrong as the setback only served to increase his determination. Unbeknownst to him, Prussia was already taking steps to abandoning its alliance with the French. On December 30, 1812, the Prussian General von Wartenberg, who led a group of 15,000 German auxiliaries in Napoleon’s army, declared a ceasefire with the Russians in the Convention of Tauroggen.
Meanwhile, Napoleon was able to raise a large army in quick time. During his campaigns, he had been able to call on his allies for support, so there was still a large number of available Frenchmen. By the end of March 1813, he had around 200,000 men marching on the Elbe. He would need these extra troops because, in February, Prussia had signed the Treaty of Kalisz with Russia which formalized their alliance. On March 17, Prussia declared war on France while Britain sent 23,000 men, along with munitions, to Russia and Prussia.