An army list, for wargaming at least, is exactly what the term implies: a list of troop types and numbers that make up one side of a wargame battle.
For players who are trying to re-enact specific historical conflicts, these lists are complied via thorough research and best represented in whatever manner the rules for the game allow. For example, an American Civil War (ACW) game re-enacting the Battle of Gettysburg would probably need to represent the storied 20th Maine Regiment. Some operational-level rules might have the 20th represented by a single stand of infantry; others may provide some sort of morale bonus to the unit due to the presence of its commander, Joshua Chamberlain. A set of tactical rules might choose to re-enact only the part of the battle connected to the 20th Maine: the battle for a hill called Little Round Top, and figures may represent individual soldiers.
But how do you compile a list for fictitious scenarios? Instead of re-enacting a known battle, what if the players just want to put some Roman Legionnaires against a horde of Visigoths? What if you are playing with Army Men and just want a showdown between green and tan troops?
In those situations – especially if you can choose whatever troop types you want to include – it may prove useful to start with a strategic theme, instead of just grabbing a handful of miniatures and deploying them on the table.