After the news of the attack on Fort Sumter reached Wisconsin, Larrabee wrote to the
Milwaukee News, a Democratic paper, to rally his party in defense of the Union. He also wrote to Wisconsin Governor
Alexander Randall and
General Rufus King to offer his services. On April 18, 1861, Larrabee enlisted as a
private in the Horicon Guard
militia, where he was elected
2nd lieutenant. The militia quickly marched to Milwaukee for enrollment in a Volunteer Regiment. A month later, on May 28, 1861, Governor Randall commissioned Larrabee as a
major for the
5th Wisconsin Volunteer Infantry Regiment. Larrabee marched with the 5th Regiment to Washington, D.C., where they eventually attached to the
Army of the Potomac. In 1862, the Regiment joined the
Peninsula Campaign. Larrabee participated in the
Siege of Yorktown, assisting in the attack on a fortress along the
Warwick River. Days later, the regiment was involved in heavy fighting at Fort Magruder during the
Battle of Williamsburg. Larrabee was commended for his work inspiring and directing the regiment during the battle. After the battle, they marched through the
Chickahominy marsh land, and Larrabee fell ill along with others in the regiment. He recuperated at
White House, Virginia, but would continue to exhibit symptoms. When new Wisconsin regiments were raised that summer, Governor
Edward Salomon appointed Larrabee to organize and command the new
24th Wisconsin Volunteer Infantry Regiment. When the 24th regiment mustered into service in August, Larrabee was promoted to
colonel. The regiment was quickly ordered to Kentucky, before much drilling or preparation. The regiment was attached to the
Army of the Cumberland, and arrived in time to join the
Battle of Perryville, which ended the
Confederate incursion in Kentucky. During the battle, the 24th was assigned to defend a cannon battery, which came under assault from a Confederate brigade, led by
Daniel Weisiger Adams. The attack was repulsed, and Larrabee was commended by his brigade commander, Colonel
Nicholas Greusel, for his leadership in the defense of the battery. After the battle, Larrabee again fell ill and recuperated in
Nashville, Tennessee, while the regiment fought at the
Battle of Stones River. In March, 1863, Larrabee returned to the regiment for a short time, but ultimately resigned in August due to his ongoing illness. ==Postbellum years==