After this Bülow went to garrison duty at Soldau. In 1802 he married the daughter of Colonel von Auer, and in the following year he became lieutenant-colonel, remaining at Soldau with his corps. The vagaries and misfortunes of his brother Dietrich affected his happiness as well as his fortune. The loss of two of his children was followed in 1806 by the death of his wife, and a further source of disappointment was the exclusion of his regiment from the field army sent against Napoleon in 1806. The disasters of the campaign aroused his energies. He did excellent service under Anton Wilhelm von L'Estocq's command in the latter part of the war, was wounded in action, and finally designated for a brigade command in Field Marshal Gebhard Leberecht von Blücher's force.