In the 1805 War of the Third Coalition, as commander of a division of dragoons, he participated in the Battle of Elchingen and later the Battle of Ulm. Six weeks later, at the Battle of Austerlitz, he made several brilliant and timely charges, including one observed by several parishioners of the town of Mönitz, who had climbed the church tower to watch the action. The French infantry had been surrounded by Austrian cavalry, which pursued them down the road. Bourcier approached from the other direction with three regiments of dragoons, having left the rest of his division behind to preserve his communication lines in Raigern. Seeing the infantry beleaguered by cavalry, he led his men in a charge, giving the infantry time to escape. His own dragoons were fired upon with cannon and grapeshot, killing or wounding several men and horses, but, as he wrote later, "[the Russian cannon fire] would have done more harm had they been directed better, being within half range."