The military alliance formed in 1949 to counter Soviet expansion was the North Atlantic Treaty Organization (NATO). Created in the early days of the Cold War, NATO consisted of Western nations committed to mutual defense against aggression, particularly from the Soviet Union. The alliance formalized a collective security arrangement, whereby an attack on one member would be considered an attack on all. NATO played a crucial role in establishing a unified military front and deterring Soviet influence in Europe. It remains an influential defense coalition today, adapting to new global security challenges.