American army, and was promoted to rank as captain 11th June of that year.
Upon the 7th July, 1838, he first entered the War Department as an assistant adjutant-general. During the Florida war he served as chief of staff to General Worth, and was in the action of Pila-Kil-Kaha on the 19th April, 1842. In 1848 he was brevetted colonel for meritorious conduct in the prosecution of his duties in connection with the Mexican war, and on the 15th July, 1852, was appointed the Adjutant-General of the United States army, General Winfield Scott being then its Commander-in-Chief.
Whilst in the United States army, he compiled his work entitled "Tactics for the Militia," a book at one time in almost universal use among the volunteer soldiery, and extensively known as "Cooper's Tactics."
In 1827 General Cooper married a daughter of General John Mason, of Clermont, Fairfax county, Virginia, and a grand-daughter of George Mason, of Gunston, "the Solon and the Cato, the lawgiver and the stern patriot of the age in which he lived," and to whose memory the constitution of Virginia and her bill of rights are lasting monuments.
At the head of the Adjutant-General's Department, United States army, General Cooper gave great satisfaction. His qualifications and his ability as an officer, and his private worth as a man, was universally acknowledged by army officers, many of those living to-day giving testimony that he was the best chief of that department the army ever had.
On the 17th March, 1861, he resigned his commission as an officer, having served the United States with a steady faithfulness and a firm adherence to all of her interests for forty-six years. In view of the fact of General Cooper's Northern birth, this step has been the subject of much comment, and some adverse criticism. His Northern friends profess to see no reason why a soldier born in their section, holding a high office of trust for life, honored and respected, should, after forty-six years' service, and in the sixty-third year of his life, relinquish a position in which he would not be called upon for field service, and cast his fortunes and tender his services to the Confederate Government. It has been said by them that he was more guided by the counsels of his friend, the Hon. Jefferson Davis, and his brother-in-law, Hon. James M. Mason, than by his native and natural opinion and belief. To those holding such sentiments, it may be truly said they did not
