A scene nearly everyone has seen before. The Hero (or maybe The Big Guy on the Five Man Band) is facing up to will large horde of Mooks ands tell their leader they're going down. The leader laughs and says, "Yeah? You and What Army?", only to then discover that the hero doesn't need one. This one person is the army, a powerhouse able to do far more than pull his own weight.
In video games, it is not only possible but routine for one man to go around and kill literally thousands of people. No one ever finds this particularly unusual, nor does anyone ever wonder about their psychological state, which would certainly be affected by taking so many lives. The impact of this is lessened if the enemies are Faceless Goons or don't register as human beings who exist solely to provide a Mook Horror Show, but the unlikelihood of one person wiping out entire armies of epic proportions isn't.
Often Handwaved by making the character a Super Soldier. In a series aimed at younger audiences, they may simply defeat large masses of people rather than outright kill them. In playing this trope, it is useful to have The Evil Army try to Zerg Rush said character in The War Sequence.
Often goes hand-in-hand with It's Up to You. When the One-Man Army is intentionally sent out to take care of the problem by himself, its a case of One Riot, One Ranger. Villains tend to underestimate this person at first, considering him Just One Man. Compare and contrast Person of Mass Destruction, who's symbolically treated like a weapon by the setting, and Omnicidal Maniac, who is all too willing to push him- or herself into this territory. In the latter case, the Omnicidal Maniac often gains his/her/its kill-count through using WMDs on hapless civilians rather than personally fighting enemy combatants.
Of course, being a One-Man Army usually qualifies you as a Badass and Crew of One. However, very few people will respect your ability. Or worse, seek to abuse it because "We Do the Impossible" is in effect. Contrast with the Badass Army where each individual could be considered this but are part of said army, as well as the Easily-Conquered World where it's the severely, severely outnumbered enemies who are kicking ass and taking names. If one man makes the army, then you have The Minion Master. Likewise, if one man becomes the army, it's Me's a Crowd.
While Real Life examples do exist, this is generally not a good idea in Real Life: the reason military forces tend to work in groups or teams at the very smallest and why police officers tend to call for backup before approaching a dangerous situation is simply that one person is only capable of so much, and may be so focused on one part of what is happening that he or she will have Failed a Spot Check on another, possibly far more dangerous one.
Also, in the case of modern combat theaters, it's more difficult to pull off if any army in the theater has airstrike capabilities, because no One-Man Army can stand up to a bunker-buster bomb. Or, if any force in the theater has nuclear capabilities and delivery capabilities.
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