With the outbreak of the Civil War in early 1861, Willich actively recruited German immigrants in the southwestern Ohio region. He joined the 9th Ohio Infantry ("Die Neuner") as regimental adjutant with the rank of first lieutenant, and was promoted to major in August of that year. He served in western Virginia, seeing action at the Battle of Rich Mountain. Willich then returned to the Ohio River valley over the winter and resumed his recruiting activities. Governor Oliver P. Morton commissioned Willich a colonel of the 32nd Indiana Infantry Regiment, also called the First German, (an all-German regiment).