Born in Newburg, New York, in 1829. He graduated at Princeton College in 1848, studied law and was admitted to the bar in 1851. He came to Iowa in 1853, locating at Keokuk where he entered upon the practice of law in partnership with Ralph P. Lowe, afterwards Governor of the State. He was elected to the House of the Seventh General Assembly in 1857 on the Democratic ticket. When the War of the Rebellion began he was commissioned major of the Fifteenth Iowa Infantry. He was in command of the regiment at the Battle of Corinth and was soon after placed on the staff of General McPherson. After the Battle of Atlanta he was promoted to Brigadier-General and at the close of the war was brevetted Major-General. He was offered a commission in the regular army but preferred to return to civil life. General Belknap had become a Republican, supporting Lincoln for President in 1864 and in 1866 was appointed Collector of Internal Revenue for the First District. When General Grant became President, General Belknap was invited into his Cabinet as Secretary of War, where he served seven years, resigning in March, 1876. Charges of official misconduct had been preferred against him by the House of Representatives in a time of great political bitterness, but in the trial by the Senate he was acquitted. Judge George G. Wright, who was a member of the Senate from Iowa, pronounced his acquittal just and his opinion was heartily indorsed by the people of Iowa who never lost confidence in the gallant officer. General Belknap died at Washington, October 13, 1890, and was buried in the National Cemetery at Arlington. Hugh J., a son of General Belknap, became a member of Congress from Chicago.