In May he received his commission as captain in the United States cavalry. Employed at first in recruiting and then in drill, he gave himself up to the study of tactics and the science of war. The career above all others to which he was suited had come to him. The field, at last, lay open before him, where all his great qualities of mind and hearthis high courage, his power of leadership and of organization, and his intellectual powers could find full play. He moved rapidly forward, just as he had already done in college and in business. His regiment, in 1862, was under Stoneman in the Peninsula, and was engaged in many actions, where Lowell's cool bravery made him constantly conspicuous. At the close of the campaign he was brevetted major, for distinguished services at Williamsburg and Slatersville.

In July, Lowell was detailed for duty as an aid to General McClellan. At Malvern Hill and South Mountain his gallantry and efficiency were strongly shown, but it was at Antietam that he distinguished himself most. Sent with orders to General Sedgwick's division, he found it retreating in confusion, under a hot fire. He did not stop to think of orders, but rode rapidly from point to point of the line, rallying company after company by the mere force and power of his word and look, checking the rout, while the storm of bullets swept all round him. His horse was shot under him, a ball passed through his coat, another broke his sword-hilt, but he came off unscathed, and his service was recognized by his being sent to Washington with the captured flags of the enemy.