In September 1845 he went with his regiment to join the forces of General Taylor in Mexico; there he took part in the battles of Palo Alto, Resaca de la Palma and Monterey, and, after his transfer to General Scott's army, which he joined in March 1847, served at Vera Cruz, Cerro Gordo, Churubusco, Molino del Rey and at the storming of Chapultepec. He was breveted first lieutenant for gallantry at Molino del Rey and captain for gallantry at Chapultepec. In August 1848, after the close of the war, he married Julia T.
The Mexicans in the battles of Palo Alto (May 8th) and Resaca de la Palma (May 9th).
Operations began early in May 1864, and five days of manoeuvring and skirmishing about Resaca and Rocky Face ended in Johnston's retirement to Resaca.
They were, however, defeated at Palo Alto (May 8) and Resaca de la Palma (May 9).
He was in command of his army at the actions of Resaca, Dallas,Kenesaw Mountain and the battles about Atlanta.
The latter was easily driven from the field, but on the following day threatened Taylor's advance in a much stronger position, Resaca de la Palma (about 4 m.