He was adjutant-general of New York state in 1839-1843, and became a brigadier-general of volunteers in the Union army in 1861, commanded a division in Virginia in 1862-1863, and, being compelled by ill health to resign from the army, was U.S. minister to the Papal States in 1863-1867.
Entering the Union army in 1861, he took part in the battles of Shiloh, Corinth and Vicksburg, as major of the 15th Iowa volunteers.
It was the central point of one of the greatest battles of the Civil War, fought on the 2nd and 3rd of May 1863, between the Union Army of the Potomac under Major-General Hooker, and the Confederate Army of Northern Virginia under General Lee.
On the 16th the Union army deployed in front of him, again over-lapping his left flank, and although a frontal attack was repulsed, the extension of the Federal right wing compelled Hood to extend his own lines more and more.
He entered the Union army in June 1861 (commission May 14) as captain of the 3rd (afterwards 6th) U.S. cavalry; on the 15th of April 1863 he became colonel of the and Massachusetts cavalry; he was wounded fatally at Cedar Creek on the 19th of October 1864, when he was promoted brigadiergeneral of U.S. Volunteers, and died on the next day at Middletown, Va.
Cooper was an early advocate of the emancipation and the enlistment in the Union army of Southern negroes, and he upheld the administration of Lincoln.
Rosecrans with Chattanooga as his objective moved from Nashville upon General Braxton Bragg, who left the winter quarters he had established at Murfreesboro and met the Union army on Stone river immediately north of Murfreesboro, on the last day of December.
As Hardee's attack rolled up the Union army from left to right, the remainder of the Confederate army was to issue from the Atlanta fortifications and join in the battle.
On the night of the 3rd Bragg withdrew and the Union army occupied Murfreesboro.
The total Federal loss (including the garrisons at Winchester and Martinsburg) amounted to 44 killed (the commander was mortally wounded), 12,520 prisoners, and 13,000 small arms. For this terrible loss to the Union army the responsibility seems to have been General Halleck's, though the blame was officially put on Colonel Miles, who died immediately after the surrender.