What is the formalized arrangement between countries to help each other in case of attack?

The formalized arrangement between countries to assist each other in case of an attack is known as a mutual defense treaty or alliance. Such agreements typically outline the obligations of each party to support one another, often through military assistance, in the event of aggression against one of the members. Examples include NATO (North Atlantic Treaty Organization) and various bilateral defense agreements. These treaties aim to deter potential aggressors by demonstrating a united front among allied nations.