Their Horses Climbed Trees tells the little known story of the five hundred volunteers from California known as the "California Hundred and Battalion", who fought in the East during the Civil War years 1863-1865, as a part of the Second Massachusetts Volunteer Cavalry Regiment. They served from 1862, when they departed from San Francisco, to the war's end at Appomattox in 1865. The unit interacted primarily against Colonel John Mosby. In the latter part of the War they served under General Sheridan in the Shenandoah Valley and at times fought alongside George Custer's cavalry. The book is mainly a compilation of newspaper accounts from 1862-1865, and 1866-1937. Also included as primary sources are letters, diary entries and excerpts from regimental documents in the National Archives. This is not a narrative or reworded history, the words are from those who were there. Their Horses Climbed Trees contains the rosters for the five hundred original volunteers, as well as obituaries for many of the veterans. Period photographs of the Company officers and a bibliography are provided. Genealogists, teachers, researchers, and historians will gain new insights into California's involvement in the Civil War in the East, which has been largely overlooked.