During Napoleon's Invasion of Russia in 1812, Zajączek commanded the 16th Infantry Division of the V Corps in la Grande Armée. His division performed well at the battle of Smolensk in August where he was wounded, recuperating till October. He rejoined his unit in the time to take part in the battle of Tarutino. After Prince Poniatowski, commander of the V Corps, was wounded, from 1 November Zajączek took charge of that formation. He led it at the battle of Vyazma and battle of Krasnoi until a wound at Berezina claimed his leg, which was amputated by doctor Dominique Jean Larrey. While recuperating from the wound he was taken prisoner by the Imperial Russian Army in Wilno (Vilnius).