Mr. Hall, upon his admission to the bar, opened an office in Dover, and commenced practice. In the spring of 1859, just before the state election, in view of the great crisis coming upon the country, at an immense meeting in Dover, he (as did also Judge Charles Doe) withdrew from the Democratic party and cast in his allegiance with the Republicans. With them, where his conscience and political principles alike placed him, has his lot been cast ever since; and it is not improbable that that one addition, in later and critical years, turned the scale in New Hampshire's political destinies. It was an episode in his life that in 1859 he was appointed, by the governor and council, school commissioner for Strafford county, and re-appointed in 1860. His early training in the country district school, his work as master in the winters, and his hard-earned higher education qualified him eminently for the practical duties of this office. In the autumn of 1861, Mr. Hall was appointed secretary of the United States senate committee to investigate the surrender of the Norfolk navy-yard. This committee consisted of John P. Hale, Andrew Johnson, and James W. Grimes. Soon after, he was appointed clerk of the senate committee on Naval Affairs, at Washington, of which Mr. Hale was chairman. He served in this capacity until March, 1862; but he wished for more immediate participation in the great struggle then in progress. The conflict, which had its symptoms in the Lecompton strife, had become war, and the young man who had then sacrificed office for principle was ready for a still greater sacrifice. In March, 1862, he was commissioned aid-de-camp and captain in the regular army of the United States. He was assigned to duty with Gen. John C. Fremont; but before he had time to join that officer, Gen. Fremont had retired from command, and Capt. Hall was transferred to the staff of Gen. A. W. Whipple, then in command at Arlington Heights, of the troops and works in front of Washington, on the south side of the Potomac. In September, 1862, a few days after the battle of Antietam, Gen. Whipple joined the army of the Potomac, and eventually marched with it to the front of Fredericksburg. On the 13th of December, 1862, he was in the battle of Fredericksburg, crossing the river with the third corps, and taking part in the sanguinary assault upon the works which covered Marye's Heights.