Recruiting an army dates back to earlier editions of D&D.
Dungeons & Dragons originated in tabletop wargaming, and players sometimes developed a series of battles as a campaign, the simulation of which extended to the conquest of real-world or fictional territory, the establishment of castles, and the hiring of troops. This concept transferred naturally into D&D.
In the original 1974 D&D rules, characters could hire men-at-arms and other forces, at a price. Clerics who reached a certain level could build a stronghold and acquire faithful followers who serve at no cost, while fighters could similarly acquire troops and establish themselves as a baron.
In AD&D 1st edition, a fighter who attains 9th level may establish a freehold, automatically attracting a body of men-at-arms, though he must still pay them a monthly fee. The cleric likewise can establish a religion stronghold.
You similarly see rules like this in the AD&D 2nd edition Player's Handbook, where the 9th level fighter attracts both low-level men-at-arms and an elite bodyguard ("bodyguard" in this case meaning a unit of troops, not just one individual). A table allows the fighter to roll to see what sort of units he might recruit. The cleric still gets to build a stronghold and can attract loyal followers.
D&D 3.0 removed the fighter and cleric's ability to acquire minions, but instead used the feat system to open this ability up to characters of all classes.