In the Fire Emblem series, some enemy units may appear in the middle of a battle. These new enemies are known as Reinforcements. Reinforcements may appear after a certain number of turns have elapsed, or after player units cross a certain point on the map. If the player knows where reinforcements are expected to appear, he/she can position their units on the squares or forts where the reinforcements will come from. This tactic will prevent the reinforcements from spawning. However, this tactic does not work in Fire Emblem: Path of Radiance or Fire Emblem: Radiant Dawn. In those two games, reinforcements will simply appear on the next open square. Reinforcements can make some maps difficult, especially if the player does not know when they will appear or how many there are. But, they can also provide player units with large amounts of experience points.
Sometimes, allied or neutral reinforcements may appear as well. In Chapter 17 of Path of Radiance, the player can call in two reinforcements for each of the last three parts. In that same game, Tanith can use her Reinforce skill to summon a trio of two Pegasus Knights and one Falcon Knight as Partner units. Tanith may only do this twice per chapter.
In Fire Emblem: Genealogy of the Holy War, it is possible for enemy Commanders to return to their home castle to spawn reinforcements if all of the units under their command are defeated. The troops assigned to a particular Commander will have this information listed in the Character Information screen. The enemy Commander must enter their home castle, and on the following turn they will exit the same castle with every unit that was under their command returning to the battlefield at full strength. There is no limit to the amount of times an enemy Commander can repeat this action, and they will immediately prioritize returning to their home castle for reinforcements if every unit under their command is defeated. This behavior can be exploited to give your units as much experience as you want.
It's worth noting that while retreating to their home castle for reinforcements, enemy Commanders will not initiate any attacks. Even if you surround them, they will simply pass their turn. This can be useful in recruiting Hannibal by leaving a character in front of Kapathogia Castle, making it impossible for him to return and thus much easier for Coirpre/Charlot to talk to him.