The book "Ergodentheorie", most of which was written when Hopf was still at the Massachusetts Institute of Technology, was published in 1937. In that book, containing only 81 pages, Hopf presented a precise and elegant summary of ergodic theory. In 1939, he established ergodicity of the geodesic flow on compact manifolds of constant negative curvature. In 1940, he was on the list of invited lecturers to the International Congress of Mathematicians to be held in Cambridge, Massachusetts. Because of the start of World War II, however, the Congress was cancelled.