The history of international mathematical co-operation over the last hundred years - from the first international congress in 1897 to plans for the World Mathematical Year 2000 - is a surprisingly compelling story. For reflected in the history of the International Mathematical Union (IMU) is all the strife among world powers, as well as aspirations for co-operation among nations in an increasingly interdependent world. As early as the 1920s, the IMU embraced principles of political neutrality, inviting every national mathematical organisation to join, and this principle of non-discrimination, while sometimes sorely tried, has held the IMU in good stead. A number of issues - the Cold War, the conflict between the Peoples Republic of China and Taiwan, a divided Germany, problems in the emerging nations of Africa - at times led to attempts to influence the IMU Executive Committee in its decisions regarding membership, location of international congresses, committee assignments, handling of protests, and awarding the coveted Fields Medals. Yet throughout, the IMU has sponsored international congresses around the world, and Professor Lehtos gripping story is one of individuals, among them many of the great mathematicians of our century, united in the common purpose of advancing their science, told against the backdrop of world events.