As a horse and wagon are accelerating from rest, the horse exerts a force of 400 N

Question:

As a horse and wagon are accelerating from rest, the horse exerts a force of 400 N on the wagon(Figure 2.56). Illustrating Newton€™s third law, the wagon exerts an equal and opposite force of 400 N. Because the two forces are in opposite directions, why don€™t they cancel each other and produce zero acceleration (i.e., no motion)?