This article proposes a computational model for an emergent collective
behavior that collectively changes strategy type, such as from attack
to defense, as seen in any kind of battle. It describes the result of
an experimental simulation with multiple autonomous robots based on t
he proposed model. Our model first defines payoff functions that creat
e multiple equilibrium states, each of which corresponds to one strate
gy type. Subsequently, we attempt to model the dynamics that cause the
robots to change their choice of strategy type collectively when a sm
all number of robots change their chosen type. In these dynamics, we p
ay particular attention to how many robots, called instigators, are re
quired to make all robots eventually change their strategy type. In ad
dition, to make it easy for all robots to change their strategy type,
we provide a mechanism by which the robots themselves reduce the utili
ty of strategies.