Most high-level discussions that assumed either nuclear weapons or a mainlandinvasion of Japan would be necessary to end the Pacific war did so with theknowledge that unconditional surrender was the official Allied policy. The "a-bombs or invasion" choice was based in part on the assumption that retention ofthe Emperor would probably not be offered to Japan. Nor was a warning to Japanof the atomic bomb in the decision-makers' plans, as they considered what wouldbe necessary to end the war. These omissions made the use of the atomic bomb seem all the more necessary for winning the war without an invasion. U.S. learns of Emperor's importanceThe U.S. government was not ignorant of the importance of the Emperor to Japanese surrender.Under Secretary of State Joseph Grewhad explained this to President Truman in person on May 28, 1945. Grew had been U.S. Ambassador to Japan for 10 years prior to the attack on Pearl Harbor and was regarded as the most knowledgeable on Japan of any U.S. government official (Leahy, pg. 274). On May 28th Grew informed Truman, "The greatest obstacle to unconditional surrender by the Japanese is their belief that this would entail the destruction or permanent removal of the Emperor and the institution of the throne" (Walter Johnson, ed., Turbulent Era, Joseph Grew, Vol. 2, pg. 1428-1429). In a June 18, 1945 meeting with Truman and his military advisors, Assistant Secretary of War John McCloyargued that Japan should be permitted to retain the Emperor and should be given a warning of the atomic bomb in order to bring an earlier and less deadly surrender (Walter Millis, ed., The Forrestal Diaries, pg. 70-71; Len Giovannitti and Fred Freed, The Decision To Dropthe Bomb, pg. 134-136). On June 28, 1945, a memo from Under Secretary of the Navy Ralph Bardwas given to Secretary of War Stimson. In the memo, Bard recommended the points made by McCloy and suggested Japan be told that Russia would enter the war against them (Manhattan Engineering District Records, Harrison-Bundy files, folder # 77, National Archives; see also Martin Sherwin, A World Destroyed, 1987 edition, pg. 307-308). Bard may have also discussed this memo with Truman in early July (Alice Kimball Smith, A Peril and a Hope, pg. 52-53; altho 15 years later, Bard did not recall the meeting: U.S. News & World Report, 8/15/60, War Was Really Won Before We Used A-bomb, pg. 73). On July 2, 1945, Sec. of War Henry Stimson and Truman discussed a proposal by Stimson to call for Japan to surrender. Stimson's memo to the President advised, "I personally think that if in saying this we should add that we do not exclude a constitutional monarchy under her present dynasty, it would substantially add to the chances of acceptance". Stimson's proposed surrenderdemand stated that the reformed Japanese government "may include a constitutional monarchy under the present dynasty" (U.S. Dept. of State, Potsdam 1, pg. 889-894).