 He returned to Iowa following First Bull Run and, as Colonel, helped to raise the 1st Regiment Iowa Volunteer Cavalry.On July 18, 1862, President Abraham Lincoln appointed Warren to the grade of brigadier general of volunteers, to rank from July 16, 1862, with a command in the army in Missouri under Major General Samuel R. Curtis.In 1863 General Warren was the leading candidate before the Republican State Convention for Governor of Iowa, but by a combination of the supporters of other candidates, Warren was defeated.