Alliance, in international relations, a formal agreement between two or more states for mutual support in case of war. Contemporary alliances provide for combined action on the part of two or more independent states and are generally defensive in nature, obligating allies to join forces if one or more of them is attacked by another state or coalition. Although alliances may be informal, they are typically formalized by a treaty of alliance, the most critical clauses of which are those that define the casus feeders, or the circumstances under which the treaty obligates an ally to aid a fellow member.
Alliances arise from states’ attempts to maintain a balance of power with each other. In a system composed of a number of medium-size countries, such as that in Europe since the Middle Ages, no single state is able to establish a lasting hegemony over all the others, largely because the other states join together in alliances against it. Thus, the repeated attempts by King Louis XIV of France (reigned 1643–1715) to dominate continental Europe led to a coalition in opposition to France and eventually to the War of the Grand Alliance; and the ambitions of Napoleon were similarly thwarted by a series of alliances formed against him.