The twentieth century was marked by the proliferation of security regimes, and collective security in particular. Under a collective security arrangement, all states at either a regional or global level agree to resolve their disputes peacefully, collectively oppose acts of aggression, and actively defend those who are victims of such aggression. It is based on the premise that security is indivisible, that is, each state’s security is intricately tied to the security of others, and no nation can be completely secure so long as the territory, independence, and populations of other states are seriously threatened. However, over the past several decades, ethnic conflicts, civil wars, guerrilla insurgencies, and other forms of internal violence have dramatically increased, even as large-scale interstate wars have declined. In addition to these sources of instability and conflict, political repression and extreme human rights abuses by governments against their populations (particularly genocide and ethnic cleansing) often generate massive refugee flows, illegal arms trafficking, and the rise of paramilitary guerrilla armies, all of which could disrupt neighboring states and regional stability. Thus, the concept of security adopted by international and regional regimes over the past few decades has expanded from the threat and use of force for deterrence and enforcement to include nation- and state-building, peacekeeping, and peace-making.