Sigel played a key role in President Abraham Lincoln´s plan of courting anti-slavery, Unionist immigrants. He was commissioned brigadier general on August 7, 1861. He organized the Third Missouri Infantry and served under Brigadier General Nathaniel Lyon in the capture of the secessionist Camp Jackson in St. Louis and at the Battle of Wilson´s Creek. His finest moment came at the Battle of Pea Ridge where, under Major General Samuel R. Curtis, he commanded two divisions and personally directed the Union artillery in the demoralizing defeat of Major General Earl Van Dorn´s Confederates.

Sigel was promoted to Major General on March 22, 1862. He was defeated at the Battle of New Market on May 15, 1864. This battle was made famous by the charge of the Virginia Military Institute cadets. Relieved of field command shortly thereafter, he resigned his commission on May 4, 1865 and re-entered civilian life. He died in New York City on August 21, 1902.