Military alliance

A formal or informal agreement among states to coordinate military action or provide military support, often including mutual defense against attack.

Collective defense: A security arrangement in which an attack on one member is treated as a concern for all. Many alliances promise members a shared response to external attack.
NATO: A transatlantic military alliance founded in 1949 by North American and European states. Its collective-defense pledge is a prominent model for a modern alliance.
Collective security: A system in which states agree to oppose aggression against any member, rather than defend only designated allies. It aims at a general response to aggression, unlike an alliance's selected commitments.
Balance of power: A distribution of power among states that prevents any one from dominating the others. Alliances can combine capabilities to counter a stronger state or rival bloc.
Alliance treaty: A binding international agreement that sets out military commitments between states. Treaties specify which threats trigger support and what assistance members owe.
Warsaw Pact: A Soviet-led military alliance of communist states in Europe, established in 1955 and dissolved in 1991. It organized the opposing military bloc during much of the Cold War.
Non-aggression pact: An agreement between states not to attack one another, sometimes including neutrality or consultation provisions. It restrains conflict between signatories without promising military aid to either.
Alliance formation: The process by which states create or join agreements for security and cooperation. This broader process explains why military alliances arise and change.
Deterrence: The prevention of an action by making its anticipated costs unacceptable to the actor. An alliance can discourage attack by making an adversary expect a collective response.
ANZUS: A 1951 security treaty among Australia, New Zealand, and the United States. Its changing membership illustrates how alliance obligations can evolve.

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