In a conversation that Ōshima had with Heinrich Himmler on 31 January 1939, the former expressed the hope that German-Japanese co-operation in the field of intelligence would lead eventually to the disintegration of the Soviet Union. Ōshima was instrumental in the forging and signing of the Anti-Comintern Pact on 25 November 1936 and the Tripartite Pact on 27 September 1940. However, on August 25, 1939, the German government decided to conclude the Molotov–Ribbentrop Pact and to suspend negotiations on a Japan-German alliance and defense agreement. This caused great turmoil in the Japanese political work, contributing to the collapse of the Hiranuma Cabinet. Ōshima was recalled to Japan (with Saburō Kurusu succeeding him) to take responsibility in September 1939, and was dismissed as an ambassador on December 27. 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." Ōshima's close relationship with Hitler and Ribbentrop gave him unparalleled access for a foreigner to German war plans and national policy, comparable to that of Winston Churchill with the American war leadership. In turn, Hitler admired the militaristic Japanese and made Ōshima a personal confidante. Ōshima made visits to the Eastern Front and the Atlantic Wall, and he met periodically with Hitler and other Nazi leaders. Being a meticulous military officer in training, he wrote detailed reports of the information provided to him by the Nazis. The reports were sent by radio to Tokyo in the Purple diplomatic cipher. Unknown to the Japanese, the PURPLE cipher was broken by American codebreakers in 1940. Thus, Oshima's reports were read almost simultaneously by his superiors in Japan and by Allied leaders and analysts as "Magic" intelligence.