As German forces attempted to breakthrough the Allied lines in the Ardennes Forest with the intent of capturing the strategic port in Antwerp, Keirsey as well as the rest of the 506th were rushed to the crossroad town of Bastogne. The 506th arrived on 19 December and was placed on the northeastern flank of Bastogne to halt the German advance from Foy. As 2nd Battalion arrived on the frontline, it dug in, preparing for an attack from the 76th Volksgrenadier Division and 2nd SS Panzer Division. On 26 December, the Germans launched a large scale assault on the American line. Keirsey stays at the Battalion Headquarters during the attack discussing the situation over a map with Hillen. Given his rank, he is likely the company commander of either Easy or Dog Companies.