An alliance is a formal agreement between two or more states or organizations to cooperate in achieving common goals or to defend against a common threat. Alliances can take many forms, including military alliances, economic alliances, and political alliances. Alliances can play an important role in shaping international events and influencing state behavior, and are often used by states as a means of balancing power and promoting their national interests.
Source: Waltz, K. N. (1979). Theory of International Politics. Reading, MA: Addison-Wesley.