Ōshima's importance for Hitler can be seen in the fact that after the conclusion of the Anti-Comintern Pact, the US Ambassador in Japan, Joseph Grew, estimated that the agreement was mainly the result of Ōshima's work, without the participation of the Japanese Ministry of Foreign Affairs. At the insistence of the Nazi government, he returned to Berlin as ambassador in February 1941, and remained in that position until the German surrender in May 1945. He dedicated his efforts to closer relations between the two countries. This including military cooperation in the Indian Ocean area (in the form of anti-merchant submarine warfare). Such was his fanatical belief in Nazi ideology that American journalist William L. Shirer, in "The Rise and Fall of the Third Reich", wrote that Ōshima was "more Nazi than the Nazis."