 Complete the following phrase:  Johnston was enthusiastic about the outbreak of the MexicanAmerican War. He served on the staff of Lt. Gen. Winfield Scott in the Siege of Veracruz, having been chosen by Scott to be the officer carrying the demand for surrender beforehand to the provincial governor. He was in the vanguard of the movement inland under Brig. Gen. David E. Twiggs and was severely wounded by grapeshot performing reconnaissance prior to the Battle of Cerro Gordo. He was appointed a brevet lieutet colonel for his actions at Cerro Gordo. After recovering in a field hospital, he rejoined the army at Puebla. During the advance toward Mexico City, he was second in command of the U.S. Regiment of Voltigeurs, a unit composed of light infantry or skirmishers. He distinguished himself at Contreras and Churubusco, was wounded again at 