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During the 2000s, the alternative between legitimacy and effectiveness was not only at the heart of the debate on multilateralism, but a driver of the evolution of multilateral practice. More limited, informal, flexible and reactive forms of international cooperation thus appeared as a response to the presumed ineffectiveness of more institutional multilateralism. However, collective action's effectiveness has more to gain from the combination of various formats than from their opposition. On the contrary, setting legitimacy and effectiveness one against the other seems to run counter to the very idea of a multilateral order, more and more challenged nowadays.