Complex sentence: After opening hostilities with a salvo of cannon fire and followed by a Mexican salvo, the American army, supported by their own battery, attacked the right flank of the Mexican defenses, commanded by a General Heredia.
Split into simpler sentences: After opening hostilities with a salvo of cannon fire and followed by a Mexican salvo. Supported by a battery of U.S. artillery, the American army then attacked the right flank of the Mexican defenses, commanded by a General Heredia.