These were some American units advancing on the town. They are up along a wall and are facing some Mexican Militia. The Militia did not hold up very well, but does any Militia? Just to the bottom of the picture, below the Americans in line, there was an American cavalry unit sweeping the flank. They managed to get into our rear before they were basically wiped out to one stand. Good tactic and very brazen. These were American volunteers. Not the regulars.