When the user engages in battle, neither the user nor the enemy can double attack. Learned by Generals at level 5.
Wary Fighter is an extremely useful skill, as it prevents the opponent from using high speed units like Swordmasters and Master Ninjas, which often do not have much strength to deal with high defense units like the General. Since most units are capable of doubling slow units, this skill will prevent them from doing so. When combined with a unit with high physical and magical defense, they can become a near-impenetrable wall where it will be very difficult to overpower them.
However, Wary Fighter can also harm the user if their speed is high enough, since they cannot double back. In addition, it is virtually useless if the opponent triggers skills like Lethality, as it will kill the user in one hit and basically negate Wary Fighter. Also, brave type weapons can still hit twice through Wary Fighter, negating the effects if skills like Dragon Fang or Rend Heaven activate.
Note: Despite the skill description, Brave weapons can perform follow up attacks regardless of whoever has Wary Fighter. However, it does prevent the fighters from attacking 4x (which happens when a Brave weapon user attacks an enemy unit with 5 or less speed then them).