[1] Underwood had served with distinction in the Mexican American War and on March 3, 1848, was offered and received a commission of Second Lieutenant in the 4th Infantry Regiment of the U. S.
Captain Underwood led a detachment from Fort Humboldt to a post at Pardee's Ranch where they escorted pack trains over the Bald Hills to protect them from the attacks by the Whilkut.
Later, Captain Underwood established Fort Gaston among the Hupa people on the Trinity River and commanded its garrison, a detachment from the U. S. 4th Infantry.
At the beginning of the American Civil War, on May 14, 1861 Underwood was promoted to Major in command of the 2nd Battalion, U. S. 18th Infantry Regiment,[2] and returned to the east to train his troops in Camp Thomas about four miles north of Columbus.
On November 30, 1861 Underwood led his battalion when the 18th Regiment was ordered to proceed to, Louisville, Kentucky and report to General Buell, who was organizing the Army of the Ohio.