In international relations, multilateralism is multiple countries working in concert on a given issue. Multilateralism was defined by Miles Kahler as “international governance of the ‘many,’” and its central principle was “opposition [of] bilateral discriminatory arrangements that were believed to enhance the leverage of the powerful over the weak and to increase international conflict.”  In 1990, Robert Keohane defined multilateralism as “the practice of coordinating national policies in groups of three or more states.
International organizations, such as the United Nations (UN) and the World Trade Organization are multilateral in nature. The main proponents of multilateralism have traditionally been the middle powers such as Canada, Australia, Switzerland, the Benelux countries and the Nordic countries. Larger states often act unilaterally, while smaller ones may have little direct power in international affairs aside from participation in the United Nations (by consolidating their UN vote in a voting bloc with other nations, for example). Multilateralism may involve several nations acting together as in the UN or may involve regional or military alliances, pacts, or groupings such as NATO. As these multilateral institutions were not imposed on states but were created and accepted by them in order to increase their ability to seek their own interests through the coordination of their policies, much of these international institutions lack tools of enforcement while instead work as frameworks that constrain opportunistic behaviour and points for coordination by facilitating exchange of information about the actual behaviour of states with reference to the standards to which they have consented.
Recently the term "Regional Multilateralism" has been proposed suggesting that "contemporary problems can be better solved at the regional rather than the bilateral or global levels" and that bringing together the concept of regional integration with that of multilateralism is necessary in today’s world.
One modern instance of multilateralism occurred in the nineteenth century in Europe after the end of the Napoleonic Wars, where the great powers met to redraw the map of Europe at the Congress of Vienna. The Concert of Europe, as it became known, was a group of great and lesser powers that would meet to resolve issues peacefully. Conferences such as the Conference of Berlin in 1884 helped reduce power conflicts during this period, and the 19th century was one of Europe's most peaceful.
Industrial and colonial competition, combined with shifts in the balance of power after the creation - by diplomacy and conquest - of Germany by Prussia meant cracks were appearing in this system by the turn of the 20th century. The concert system was utterly destroyed by the First World War. After that conflict, world leaders created the League of Nations in an attempt to prevent a similar conflict. A number of international arms limitation treaties were also signed such as the Kellogg-Briand Pact. But the League proved insufficient to prevent Japan's conquests in Eastern Asia in the 1930s, escalating German aggression and, ultimately, the outbreak of the Second World War in 1939.
After the Second World War the victors, having drawn experience from the failure of the League of Nations, created the United Nations in 1945 with a structure intended to address the weaknesses of the previous body. Unlike the League, the UN had the active participation of the United States and the Soviet Union, the world's two greatest contemporary powers. Along with the political institutions of the UN the post-war years also saw the development of other multilateral organizations such as the General Agreement on Tariffs and Trade (GATT) (now the World Trade Organization), the World Bank (so-called 'Bretton Woods' institutions) and the World Health Organization. The collective multilateral framework played an important role in maintaining world peace in the Cold War. Moreover, United Nations peacekeepers stationed around the world became one of the most visible symbols of multilateralism in recent decades.
Today there are several multilateral institutions of varying scope and subject matter, ranging from the International Telecommunications Union (ITU) to the World Intellectual Property Organization (WIPO) and Organisation for the Prohibition of Chemical Weapons (OPCW). Many of these institutions were founded or are supported by the UN.
Compared to unilateralism and bilateralism where only the country itself decides on what to do or make decisions between two nations, multilateralism is much more complex and challenging. It involves a number of nations which makes reaching an agreement difficult. In multilateralism, there may be no consensus; each nations have to dedicate to some degree, to make the best outcome for all. The multilateral system has encountered mounting challenges since the end of the Cold War. The United States has become increasingly dominant on the world stage in terms of military and economic power, which has led certain countries (such as Iran, China, and India) to question the United Nations' multilateral relevance. Concurrently, a perception developed among some internationalists, such as former UN Secretary General Kofi Annan, that the United States is more inclined to act unilaterally in situations with international implications. This trend began when the U.S. Senate, in October 1999, refused to ratify the Comprehensive Test Ban Treaty, which President Bill Clinton had signed in September 1996. Under President George W. Bush the United States rejected such multilateral agreements as the Kyoto Protocol, the International Criminal Court, the Ottawa Treaty banning anti-personnel land mines and a draft protocol to ensure compliance by States with the Biological Weapons Convention. Also under the Bush administration, the United States withdrew from the Anti-Ballistic Missile Treaty, which the Nixon administration and the Soviet Union had negotiated and jointly signed in 1972. In a direct challenge to the actions of the Bush administration, French president Jacques Chirac directly challenged the way of unilateralism: "In an open world, no one can live in isolation, no one can act alone in the name of all, and no one can accept the anarchy of a society without rules." He then proceeded to tout the advantages of multilateralism.
Global multilateralism is presently being challenged, particularly with respect to trade, by emerging regional arrangements such as the European Union or NAFTA, not in themselves incompatible with larger multilateral accords. More seriously, the original sponsor of post-war multilateralism in economic regimes, the United States, has turned to unilateral action and bilateral confrontation in trade and other negotiations as a result of frustration with the intricacies of consensus-building in a multilateral forum. As the most powerful member of the international community, the United States has the least to lose from abandoning multilateralism; the weakest nations have the most to lose, but the cost for all would be high.