Organized humanism did not begin as a single national movement that later went international. It began international, deliberately. In August 1952, delegates from five existing national societies, the American Humanist Association and the American Ethical Union from the United States, the British Ethical Union, the Vienna Ethical Society and the Dutch Humanist League, met in Amsterdam to found a federation that could speak for humanism collectively rather than leave each national body to represent itself alone.
They called it the International Humanist and Ethical Union, and at that founding congress adopted a statement of shared principles that came to be known as the Amsterdam Declaration, the document establishing what the federation actually holds humanism to mean in common, apart from whatever differences separated its founding member societies on other questions. The choice to draft a joint declaration at the very first congress, rather than deferring the question, signaled that the new federation intended to be a body with an actual shared position, not merely a coordinating office for otherwise independent national organizations.
That founding document has outlasted the organization's original name. The federation renamed itself Humanists International in 2019, but the declaration it adopted in Amsterdam in 1952 remains the reference point its later revisions return to and build on, still described in the organization's own materials as its founding statement.