A free open-access book by Norbert Schappacher describing the pre-history, internationalization and coming into being of the International Mathematical Union (in honor of its centennial year) is now available as a download here:

https://link.springer.com/book/10.1007/978-3-030-95683-7

This book is about international relations in mathematics over the last two hundred years, since the 1820s. It focusses on institutions and organizations that were created with a view to framing the international dimension of mathematical research.