  In a multi-agent system, transitioning from a centralized to a distributed
decision-making strategy can introduce vulnerability to adversarial
manipulation. We study the potential for adversarial manipulation in a class of
graphical coordination games where the adversary can pose as a friendly agent
in the game, thereby influencing the decision-making rules of a subset of
agents. The adversary's influence can cascade throughout the system, indirectly
influencing other agents' behavior and significantly impacting the emergent
collective behavior. The main results in this paper focus on characterizing
conditions under which the adversary's local influence can dramatically impact
the emergent global behavior, e.g., destabilize efficient Nash equilibria.
