What agreement involves nations defending each other in the case of an attack?

The agreement that best describes nations defending each other in the case of an attack is collective security. Collective security operates on the principle that an attack against one state is considered an attack against all member states. This system seeks to deter aggression by promising that member nations will unite and respond collectively to any threat, thereby enhancing the security of each member.

Mutual defense treaties can also involve similar obligations but are typically bilateral agreements between specific countries, rather than a broad commitment among multiple nations as seen in collective security arrangements. Peace agreements tend to focus on the cessation of hostilities, rather than mutual defense protocols. Alliance treaties can encompass mutual defense but are often broader, including economic and political commitments beyond military support. Collectively, these distinctions clarify why collective security is the most fitting answer for the scenario described.