At the outbreak of First Schleswig War, Læssøe was chief of staff at the army command. He participated in the battles at Bov, Nybøl and Dybbøl Banke. On this occasion, Læssøe came into conflict with the Minister of War, who questioned the competence of the army's leadership. And as a result of the Defeat at Egernførde, the hunt for a scapegoat began. Minister of War Hansen then used the defeat to remove the commander of the high command, General Christoph von Krogh and his chief of staff Frederik Læssøe.