In Gen. Scott's Mexican campaign he was promoted captain and received the brevet of major for gallantry at Contreras and Churubusco.

Wessells, though wounded, seized the regimental flag on the death of the color sergeant and put himself at the head of his men.

On his return from the war the state of Connecticut voted him a jeweled sword, which was presented to him with military ceremonies.

He was made a brigadier-general of volunteers April 25 and served in the Peninsula, receiving the regular army brevet of lieutenant-colonel for gallantry at the Battle of Fair Oaks, where he was wounded.

In Gen. George B. McClellan's change of base he commanded the rear guard, and then engaged in the defense of Suffolk, Virginia, afterward serving in North Carolina.

On April 17, 1864, he was attacked at Plymouth, North Carolina, where he had a garrison of about 3,000 men, by Gen. Robert F. Hoke with about 15,000 Confederate troops and the ironclad CSS Albemarle After a gallant defense which lasted three days, Gen. Wessells surrendered the town.