Which term is used for a cooperative defense pact among nations?

The term "collective security" refers to a system in which states work together to enhance their mutual defense and security. It is based on the principle that an attack on one member is considered an attack on all, thus obligating members to respond collectively to threats. This concept is prominently seen in organizations like the North Atlantic Treaty Organization (NATO), where member countries agree to defend each other in case of aggression.

In contrast, unilateral security emphasizes actions taken by a state on its own, without the support of allies. A defense initiative typically refers to a specific plan or program aimed at enhancing military capability or responding to particular threats, rather than the broader concept of mutual defense among nations. The term "international alliance" can describe various forms of agreements between countries, but it does not specifically denote the principle of collective action against common threats as clearly as collective security does. The specificity and mutual commitment inherent in collective security make it the correct answer in this context.