What describes an agreement among nations to defend each other in case of an attack?

The correct response is rooted in the concept of collective security, which refers to an arrangement where countries agree to collectively respond to threats against any one of them, effectively providing mutual defense. This principle is based on the idea that an attack on one member is considered an attack on all, thereby promoting a sense of shared responsibility and deterrence against aggression.

Collective security agreements often manifest in formal alliances or treaties that outline the obligations of member states to assist one another if one is attacked, thereby maintaining international peace and stability. Examples include organizations like the United Nations and NATO, where member nations commit to support each other in the event of military aggression.

In this context, while collective defense also involves mutual protection, it is usually tailored to specific military alliances rather than a broader system aimed at maintaining global peace. The other options, like the pact of nonaggression or mutual aid, imply different forms of diplomatic and supportive agreements that do not specifically encapsulate the comprehensive defense strategy embodied in collective security frameworks.