For centuries, the balance of power has been a confusing notion. The bottom line is, states try to ally with each other to create equality between the opposing alliances, which is necessary to preserve peace in an anarchical international system. Trying to preserve their integrity and national interests, smaller states find it necessary to cooperate with larger states to enlarge their military capabilities and make sure that other large or small states would not dare intervene. Such cooperation is called external balancing and it creates an atmosphere of distrust and competition in the international political arena. Any party can be an ally or a foe to the other party depending on the state’s needs and a current balance of power.
Though there are multiple goals for countries to pursue, survival is the primary one, and it stimulates states to gain additional military power. This is the simplest explanation of why different alliances form. Realists put two more assumptions to this thesis. First, the acquisition of power by one state means its loss for another one. And last, an aggression against a weaker state is an anticipated behavior of a stronger state that pursues greater power. In fact, states naturally think for themselves thus they seek superiority. In this competition, however, they create the system that puts them in balance with each other.
By the 1970s, the described realist paradigm faced criticism and was redefined as neorealism. Anyway, the balance of power has been and still remains the central principle of organization within the anarchical international system. There is no other way for small states to gain protection than to align with more powerful states, and the principle remains valid even in the 21st century. Balance strategy is rather a natural result of normal international relations. After the iron curtain has gone, the bipolar balance of power characteristic to the 20th century vanished. Today we observe an increasing role of soft balancing that determines international relationships.