For the film, see Second in Command.The second-in-command (2i/c or 2IC) is the deputy commander of any British Army or Royal Marines unit, from battalion or regiment downwards. He or she is thus the equivalent of an executive officer in the United States Army. This terminology is also used in many other Commonwealth armies.The second-in-command of a battalion is usually a major and the second-in-command of a regiment is a lieutenant colonel.