He moved to the United States in 1902. Theodor Herzl had suggested to Richard Gottheil that he hire de Haas as the new secretary of the Federation of American Zionists (FAZ) to replace Stephen Samuel Wise. De Haas assumed the leadership of the fragmented American Zionist movement. One of his best known relationships was his friendship with Louis Brandeis, the most widely known and admired secular Jew in America. De Haas introduced Brandeis to the ideas of Theodor Herzl and ideals of Zionism. After a relatively short period of examination and self-examination, Louis Brandeis became an ardent, committed Zionist in 1908. More importantly, Brandeis would head the FAZ and the American Zionist movement by 1912. He died at Mount Sinai Hospital in New York City on 21 March 1937 after a lengthy illness.