A Master-at-Arms (US: MA; UK & some Commonwealth: MAA) may be a naval rating, responsible for law enforcement, regulating duties, security, Anti-Terrorism/Force Protection (AT/FP) for/of a country's navy; an Army officer responsible for physical training; or a member of the crew of a merchant ship (usually a passenger vessel) responsible for security and law enforcement. In some navies, a "ship's corporal" is a position—not the rank—of a petty officer who assists the master-at-arms in his various duties. Historically, a master-at-arms was responsible for the training of soldiers during peace time, or actively involved in leading the defense of a fortification during war time.