If a captain were to address one of his men, a man named Smith, say, what would he call him? Regular Smith? Mr. Smith? Soldier Smith? I know very little about war in general and have no idea if this is a stupid question. Any help would be wonderful.
Most likely as his rank. Private Smith. Maybe just his name is there was a closeness or relaxed situation, or maybe in the interest of time or stress but unlikely between an officer and the ranks.
Mr. Smith would be if he was a junior officer and a senior officer was addressing him. Though this was usually more true in the naval services.
Throughout history using rank when addressing another serviceman serves multiple purposes. It makes it very clear who you are speaking to. Shows the proper respect and reinforces the chain of command.