Basic Trope: Owing to a sudden lack of superiors, a character finds himself in command.
Straight: Captain Smithson is assigned duty officer while all the other officers — captains, majors, colonels — go off to a concert with the country they are negotiating with. He receives a message that they were all murdered, and he must surrender.
Exaggerated: The officers' and NCOs' rations were spiked with poison. Private Jones, the senior private, takes command.
Colonel Williams, having a stomach flu, is in no shape to give orders; Major Taylor issues them.
The sergeant is shot while they are retreating from a mission; the senior private has another private carry him and gives a few orders for the stretch before they return to their own lines.
Justified: Someone in command, no matter how inexperienced, is always better than nobody in command. Besides, there is a chain of command for a reason - when units are separated from their commanders or circumstances intervene, everyone must be capable of assuming leadership.
Inverted: The officers are treacherously taken prisoner for their hostage value; their subordinates are all slaughtered so they have no one to command.
Subverted: Captain Smithson thinks that he is a superior officer over a much larger unit for a short time, but soon they regain contact with a higher-ranking commander who takes charge.
Double Subverted: Captain Smithson is soon separated from the commander again and takes charge for the rest of the story.
Parodied: After Colonel Williams is killed, Major Taylor fakes a stomach flu, and leaves Captain Smithson in command. Smithson promptly is knocked unconscious when he accidentally gets bonked in the head by a falling tree branch, and Lieutenant Jones faints in panic, leaving a nonplussed Sergeant Rock in charge.
Zig-Zagged: Captain Smithson assumes command following the loss of Major Taylor and Colonel Williams, but it turns out Captain Thompson should actually be in command, due to him having more combat experience. However, Smithson was nearest the radio operator, while Thompson was pinned down in combat elsewhere, and unable to effectively lead the unit from his position, so Smithson has command. For now.