Deterring the invasion or coercion of weak or medium-strength states that are important but not vital interests of major states is a key strategic challenge of the new era. This paper describes strategies for doing so. It begins by using decision-modeling methods to identify factors that would influence the decisions of would-be aggressors, including factors idiosyncratic to individual leaders. It then discusses how both immediate and general deterrence might be strengthened by a variety of political, economic, and military measures. The measures discussed include reasonably capable defensive forces that cannot easily be bypassed, operational arms control to make surprise attack more difficult, forward-deployed protector forces, and formal arrangements through regional security structures that would assure the long-term punishment of aggressors through political and economic isolation and, perhaps, military measures. The paper also encourages identifying and rooting out "dangerous ideas" that increase regional tensions and hatreds, and that could encourage aggression during a crisis. The following pages document the methods described here and include extensive references to relevant literature in political science, psychology, history, and strategy.