In 1933, the Roosevelt Administration extended diplomatic recognition to the Soviet Union and Henderson was assigned to Russia to help reopen the U.S. Embassy in Moscow. Aiding him in this task were fellow junior officers George F. Kennan and Charles Bohlen, who along with Henderson would later be considered the Department's top Soviet specialists. In 1935, the Kremlin broke its pledge to not to interfere in U. S. domestic politics. In response, Ambassador Bullitt returned to Washington in disgust, leaving Henderson for a time as chargé d'affaires in Moscow. As chargé, Henderson warned Washington that the Soviet Union was likely to cooperate with Nazi Germany. Four years later, Moscow signed the Soviet-German Non-aggression Pact of 1939. Henderson was one of the contributors to the Welles declaration of 1940, which established US non recognition policy of Baltic states occupation by Soviet Union.