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.