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 foederis, 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 (power, balance of) 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 (Grand Alliance, War of the); and the ambitions of Napoleon (Napoleon I) were similarly thwarted by a series of alliances formed against him.

Although typically associated with the Westphalian states system and the European balance of power, alliances have taken shape on other continents and in other eras. In his classic work Artha-śāstra (c. 300 BC), Kauṭilya (Kautilya), an adviser to Indian king Candra Gupta (Chandra Gupta), argued that in pursuing alliances countries should seek support and assistance from distant states against the menace of neighbouring ones (according to the logic that the enemy of one's enemy must be one's friend). The legacy of colonialism in Africa retarded the development of collective-defense schemes there, but elsewhere in the developing world alliances played a critical role in the evolving regional balance. For example, in the 1865–70 Paraguayan War (Triple Alliance, War of the), the Triple Alliance of Argentina, Brazil, and Uruguay devastated Paraguay, reducing its territorial possessions as well as its population by about 60 percent. Until the Cold War in the last half of the 20th century, ideology was not usually a significant factor in the formation of such coalitions. For example, in 1536 Francis I, the Roman Catholic king of France, joined with the Ottoman sultan Süleyman I, who was a Muslim, against the Holy Roman emperor Charles V, another Catholic, because Charles's possessions almost encircled France. Similarly, in World War II Great Britain and the United States allied themselves with the communist Soviet Union in order to defeat Nazi Germany.

A new level of alliance building in Europe was reached in the late 19th century, when enmity between Germany and France polarized Europe into two rival alliances. By 1910 most of the major states of Europe belonged to one or the other of these great opposing alliances: the Central Powers, whose principal members were Germany and Austria-Hungary, and the Allies (Allied Powers), composed of France, Russia, and Great Britain. This bipolar system had a destabilizing effect, since conflict between any two members of opposing blocs carried the threat of general war. Eventually, a dispute between Russia and Austria-Hungary in 1914 quickly drew their fellow bloc members into the general conflict that became known as World War I (1914–18). The war's outcome was effectively decided when the United States abandoned its traditional isolationism and joined the Allied side in 1917 as one of several “Associated” Powers.

The Allied victors sought to ensure the postwar peace by forming the League of Nations (Nations, League of), which operated as a collective security agreement calling for joint action by all its members to defend any individual member or members against an aggressor. A collective security agreement differs from an alliance in several ways: (1) it is more inclusive in its membership, (2) the target of the agreement is unnamed and can be any potential aggressor, including even one of the signatories, and (3) the object of the agreement is the deterrence of a potential aggressor by the prospect that preponderant power will be organized and brought to bear against him. The League of Nations became demonstrably ineffective by the mid-1930s, however, after its members declined to use force to stop aggressive acts by Japan, Italy, and Germany.

These three countries soon formed the Axis (Axis Powers), an offensive alliance that contested for world dominion in World War II (1939–45) with a defensive alliance led by Great Britain, France, China, and, beginning in 1941, the Soviet Union and the United States. With the defeat of the Axis Powers in 1945, the victorious Allies formed the United Nations (UN), a worldwide organization devoted to the principles of collective security and international cooperation. The UN coexisted rather ineffectively, however, with the robust military alliances formed by the United States and the Soviet Union along sharp ideological lines after the war. In 1949 the United States and Canada joined with Britain and other western European countries to form the North Atlantic Treaty Organization (NATO), and in 1955 the Soviet Union and its central and eastern European satellites formed the Warsaw Pact following West Germany's accession to NATO. The Cold War rivalry between these two alliances, which also included other treaty organizations established by the United States (e.g., the Southeast Asia Treaty Organization, the Central Treaty Organization, and the ANZUS Pact), ended with the Soviet Union's collapse and the dissolution of the Warsaw Pact in 1991.

The alliances of the Cold War were publicly acknowledged peacetime coalitions. In these respects they differed from most previous alliances, such as the partly secret German-Soviet Nonaggression Pact (1939), which was concluded less than 10 days before Germany invaded Poland and started World War II. Modern alliances generally require a joint effort far more integrated than was necessary in earlier times. For example, in the coalitions of World War II, combined agencies for military and economic planning were a common and conspicuous feature. Even in less tightly knit alliances, such as NATO (North Atlantic Treaty Organization), great importance was attached to close and cooperative action, both military and political, particularly in maintaining the West's strategy of nuclear deterrence and in managing conflicts in regions on the European periphery, such as the Balkans.

In the aftermath of the Cold War and in the absence of clear European blocs at the beginning of the 21st century, scholars and policymakers debated whether alliances required an enemy to remain cohesive. For example, some policymakers argued that there was no justification for NATO's continued existence given the disappearance of the Soviet Union. In contrast, others claimed that the organization could and should evolve to play an increased role in conflict management on Europe's troubled periphery, particularly in the Balkans. Nevertheless, various high-profile crises underscored the traditional approach to alliance making. For example, following the terrorist attacks in the United States on the World Trade Center and the Pentagon on September 11, 2001, the U.S. administration of President George W. Bush (Bush, George W.) forged a diverse coalition comprising a variety of old (e.g., the United Kingdom) and new (e.g., Uzbekistan) partners to combat international terrorism.