Sweeny was commissioned brigadier-general of volunteers, Nov. 29, 1862. In the Atlanta campaign, he commanded the Second Division of the Sixteenth Corps, Army of the Tennessee. At Snake Creek Gap he took possession of the “Gap” with his division twenty-four hours before the arrival of supporting cavalry, and held it in spite of the desperate efforts of the enemy to dislodge him. Subsequently he took part in the battle of Resaca, and forced a passage across the Oostenaula river, at Lay’s Ferry, where he fought a successful battle which resulted in Gen. “Joe” Johnston’s retreat southward. He also took part in the battles of Dallas and Kennesaw Mountain, the actions at Nickajack Creek, Ruff’s Mills, Rome Cross Roads, Calhoun’s Ferry and other engagements. At the battle before Atlanta, July 22, 1864, his division drove the enemy back with great slaughter, capturing four battle flags and nine hundred prisoners. This was the day that General McPherson was killed—he had shared Sweeny’s tent the previous night. At the conclusion of the battle Gen. Frank P. Blair, commanding the Seventeenth Army Corps, who had witnessed the repulse by Sweeny’s division, rode down at the head of his staff to where Sweeny stood and grasping him warmly by the hand, said: “Sweeny, I congratulate you. You have saved the army of the Tennessee!” General Sweeny was one of the Guard of Honor in charge of the remains of President Lincoln when they lay in state in the city hall, New York. On Aug. 24, 1865, Sweeny was mustered out of the volunteer service, and was in command of the post at Nashville, Tenn., to October of that year. An ardent lover of the land of his birth, General Sweeny longed for the day and the hour when, on the field of battle, he might meet the common foe of his adopted and of his native land. Even while lying disabled by his wounds in Mexico, he had made arrangements to cast himself into the then expected struggle for Irish independence, but the opportunity was not afforded him. In 1856, while he was stationed at Fort Pierre, Nebraska territory, he wrote to his family: “I see they are making another movement for Irish independence. I hope it will amount to something this time. E——, how would you like me to embark in such an undertaking? We might accomplish great things—do deeds that our children could point at on the page of history with pride—perhaps help to pull a sinewy tyrant from his throne, and raise a prostrate people from chains to liberty.”