Collective defense is a security arrangement where a group of nations agrees to protect each other in case one of them is attacked. This means that if an enemy targets one member, all members will respond together to defend it. A well-known example of this is NATO, where countries in North America and Europe stand united against threats. The idea behind collective defense is that an attack on one is seen as an attack on all, which helps deter potential aggressors. By working together, nations can share resources, intelligence, and military strength, making them more secure than if they acted alone.