International alliance refers to the formal agreements among sovereign states of various countries to cooperate for various reasons. These alliances may be primarily military in nature such as NATO or political unions like the European Union or they may be in the field of resource or cultural alliances. Most alliances of states have objectives in multiple fields.
According to the most widely held explanation, countries seek alliances for the primary purpose of enhancing their effective military capabilities by combination with others. In this explanation the most important factors are the balance of power and the threat from a common external enemy. This explanation has become a central part of the realism school of thought in world politics.
However, other explanations of the origin of alliances have emerged from other schools of thought in world politics. The Indian king Chandragupta, for example, was an advocate of the alliance strategy in his classic Artha-shastra (The Science of Material Gain).
Another important explanation is that states pursue alliances to achieve their domestic goals, such as gaining resources, prestige, or power. This explanation is often associated with the idea of the “necessary state” which states need in order to achieve their objectives.
In the immediate post-Cold War period, a key element in NATO’s efforts to reimagine itself was to form relationships with former Soviet and Eastern European countries. This was a first step in demonstrating to Russia that the organization did not intend to disappear after the end of the Cold War. The Partnership for Peace program, while not guaranteeing future NATO membership, provided the opportunity to work together with countries from that region on multilateral projects like humanitarian assistance, peacekeeping, and crisis management.