In international relations, multilateralism refers to an alliance of multiple countries pursuing a common goal. Multilateralism is based on the principles of inclusivity, equality, and cooperation, and aims to foster a more peaceful, prosperous, and sustainable world. Middle powers play a crucial role in the international system by promoting multilateralism and internationalism.

One of the key advantages of multilateralism is that it enables countries to solve problems that transcend national boundaries, such as climate change, terrorism, and pandemics, through shared responsibility and burden-sharing. However, multilateralism has challenges. The rise of populism, nationalism, and protectionism in some countries has raised concerns about the future of multilateralism and the effectiveness of international cooperation.

Multilateralism, in the form of membership in international institutions, serves to bind powerful nations, discourage unilateralism, and gives small powers a voice and influence that they could not otherwise exercise. For a small power to influence a great power, the Lilliputian strategy of small countries banding together to collectively bind a larger one can be effective. Similarly, multilateralism may allow one great power to influence another great power. For a great power to seek control through bilateral ties could be costly; it may require bargaining and compromise with the other great power.

Miles Kahler defines multilateralism as "international governance" or global governance of the "many", and its central principle was "opposition [to] bilateral discriminatory arrangements that were believed to enhance the leverage of the powerful over the weak and to increase international conflict."; Robert Keohane defined it as "the practice of coordinating national policies in groups of three or more states."

John Ruggie further elaborated the concept in his influential writings on multilateralism. Based on principles of "indivisibility" and "diffuse reciprocity he defined it as "an institutional form which coordinates relations among three or more states based on 'generalized' principles of conduct ... which specify appropriate conduct for a class of actions, without regard to particularistic interests of the parties or the strategic exigencies that may exist in any occurrence." He further clarified that multilateralism is 'a unique product of US global hegemony [. . . ] not necessarily a post-war American invention', but a reflection of post-war 'American hegemony'.

Embedding the target state in a multilateral alliance reduces the costs borne by the power-seeking control, but it also offers the same binding benefits of the Lilliputian strategy. Furthermore, if a small power seeks control over another small power, multilateralism may be the only choice, because small powers rarely have the resources to exert control on their own. As such, power disparities are accommodated to the weaker states by having more predictable bigger states and means to achieve control through collective action. Powerful states also buy into multilateral agreements by writing the rules and having privileges such as veto power and special status.^{[citation needed]}

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. These multilateral institutions are not imposed on states but are created and accepted by them to increase their ability to seek their own interests through the coordination of their policies. Moreover, they serve as frameworks that constrain opportunistic behaviour and encourage coordination by facilitating the exchange of information about the actual behaviour of states regarding the standards to which they have consented.

The term "regional multilateralism" has been proposed by Harris Mylonas and Emirhan Yorulmazlar, 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. Regionalism dates from the time of the earliest development of political communities, where economic and political relations naturally had a strong regionalist focus due to restrictions on technology, trade, and communications.