First of all, an alliance contributes to the balance of power in a given setting. Defensive alliances deter aggressive states from using military force.

A formal military alliance such as NATO usually has the purpose of collective defense, where its members pool their capabilities and launch a collective effort to protect one another against a potential outside aggressor.

Moreover, alliances help contain potential rivals. For example, the US has benefited from the alliances to contain the Soviet Union (later Russia), China, Iran, and other states that are perceived to be threatening the American political, military, and economic interests. Alliances also provide the means to keep the allies under control through restricting their interaction with potential rivals.

Furthermore, alliances enhance the military power of its members. The military strength of an alliance naturally enhances the national power in terms of personnel, military equipment, defense spending, high technology, intelligence sharing, and financial resources.

Finally, alliances provide the members with cost-effective options in pursuing national security interests through pooling the military capabilities and sharing the financial burdens. Otherwise, the members would be forced to extensively increase their defense spending and conscript many people that may ultimately strain the workforce. For minor powers, alliances have increasingly become a means not only to acquire essential military benefits but also to achieve various nonmilitary objectives such as diplomatic support, technology transfer, economic aid, and trade privileges.