States sometimes formalize their cooperation through treaties that commit them to mutual support. An alliance is such an agreement, usually focused on security but sometimes extending to economic or political coordination.

Alliances can be bilateral or multilateral. They may obligate members to consult, to share intelligence, or to treat an attack on one as an attack on all. NATO remains the most prominent contemporary example of a collective-defense alliance. Historical alliances have shaped the outcomes of wars and the balance of power for centuries.

Membership brings benefits and costs. Allies gain deterrence and pooled resources; they also accept constraints on unilateral action and the risk of being drawn into conflicts not of their own making. Alliances evolve, expand, or dissolve as interests and threats change. Diplomats spend considerable effort managing the expectations and burdens that alliance commitments create.

• Formal agreement for mutual support
• Often centered on security guarantees
• Can be bilateral or multilateral
• Shapes strategic behavior of member states