When Napoleon marched on Moscow in the summer of 1812, he left Marshal Oudinot's corps - later commanded by Laurent Gouvion Saint-Cyr - to hold a defensive bastion at Polotsk on the Dwina River. The position was critical for several reasons. It kept Wittgenstein's Russian army from marching south to threaten the rear of the Grande Armee around Moscow. It protected Vitebsk, one of three massive supply depots Napoleon had stockpiled in Belarus to fuel a winter campaign if Moscow proved indecisive. The other two were at Minsk and Smolensk. Through summer and early fall, the line held. Saint-Cyr fought Wittgenstein to a draw at the First Battle of Polotsk in August, and from Napoleon's perspective, that was a victory: Wittgenstein had been kept north of the Dwina, away from the supply columns and away from the Grande Armee's exposed rear.

By mid-October, Russian reinforcements had transformed the situation. Wittgenstein commanded close to 50,000 men - 31,000 regulars and 9,000 militia at Polotsk itself, plus 9,000 more under General Steingall maneuvering toward the French rear. Against this, Saint-Cyr could field at most 27,000 troops, many of them Bavarians weakened by typhus and exhaustion. Napoleon, by then deep in the burned-out husk of Moscow, had no reinforcements to send. The Russian advantage was no longer marginal; it was decisive. Wittgenstein opened his offensive on 18 October.

The first day's fighting was brutal and inconclusive. Russian columns made seven consecutive frontal assaults on the Polotsk defenses while Steingall pressed in from the east. All seven attacks were beaten back by nightfall, but at heavy cost - Russian losses came to 8,000 to 12,000 men, and French losses to about 8,000 of a far smaller force. Saint-Cyr could claim a tactical win on day one. He could not hold the position. Wittgenstein kept up an artillery bombardment overnight, and much of Polotsk burned. Late on 19 October, Steingall closed to within four miles of the town, and Saint-Cyr realized he was being encircled. That night, with the position untenable, he began evacuating across the Dwina. House-to-house fighting filled the streets as the Russians launched their final assault.

On the morning of 20 October, Saint-Cyr had one immediate problem: Steingall sat astride his line of retreat. He ordered his Bavarian contingent - the same Bavarians who had been worn down by months of garrison disease and combat - to drive Steingall back. They did. The Bavarian counterattack was severe enough to push Steingall into a retreat with heavy casualties of his own, opening the southern road. Saint-Cyr's surviving 15,000 troops escaped encirclement and fell back. Polotsk itself was lost. Two weeks later, Wittgenstein's army captured the supply depot at Vitebsk - the inland reserve Napoleon had been counting on for the winter - and the Dwina Line ceased to exist as a coherent French position.

Wittgenstein's victory mattered less for the ground it took than for what it released. With his northern flank gone, Napoleon could no longer count on a defensible line behind which the Grande Armee could winter. The road to the catastrophe at the Berezina opened directly from the wreckage at Polotsk. In late November, three Russian armies converged on Napoleon from separate directions at the Berezina river crossings - Wittgenstein's from the north, Admiral Chichagov's from the south, Kutuzov's main pursuit from the east. The Grande Armee, a third of its strength dead from typhus and frost and Cossack raids since Moscow, lost most of what remained at the crossings. The Second Battle of Polotsk was the first major Russian offensive victory of 1812. It was the moment when the war stopped being something Napoleon was fighting and became something happening to him.