Emperor Hirohito cited damage from the attacks, inadequate preparations to resist invasion and the Soviet offensive as his justifications for authorizing the surrender. To achieve this, the American Twentieth Strategic Air Force, in concert with its Allies, dropped 160,800 tons of bombs on the Japanese home islands. Of this total, 147,000 tons of bombs were dropped by the B-29 bomber force. Around 90 percent of the American tonnage fell in the last five months of the war. The financial cost of the campaign to the United States was $4 billion; this expenditure was much lower than the $30 billion spent on bomber operations in Europe, and a small proportion of the $330 billion the US Government spent on the war. There has been debate over the morality of the air campaign against Japan since World War II. During the war the American public approved of the bombing of Germany and Japan, and the few people who criticized the raids were seen as unrealistic or even traitors. Some United States government and military personnel believed that the bombing campaign was morally ambiguous, however, but rarely voiced their views publicly. The moral concerns over the attacks have focused on the large number of civilian casualties and property damage they caused. For this and other reasons, British philosopher A. C. Grayling has concluded that the Allied area bombing campaigns against both Japan and Germany constituted moral crimes. Mark Selden described the summer 1945 peak of the bombing campaign as "still perhaps unrivaled in the magnitude of human slaughter" and stated that the factors contributing to its intensity were a combination of "technological breakthroughs, American nationalism, and the erosion of moral and political scruples about killing of civilians, perhaps intensified by the racism that crystallized in the Pacific theater". Edwin P. Hoyt wrote in 1987 that Japanese people commonly regard the Allied bombing of civilians as the worst atrocity of the war. It has also been suggested that anti-Japanese sentiment was a factor motivating the USAAF's emphasis on firebombing during the campaign against Japan while most of its raids on Germany used precision bombing tactics. However, historian Richard B. Frank argues that this difference was attributable to the evolution in views towards bombing over the course of the war, the limited intelligence on the structure of the Japanese economy available to the Allies and the much greater vulnerability of Japanese cities to incendiary bombs. The moral defense of the attacks on Japanese cities rests on an argument that they saved lives by shortening the war. The USSBS concluded that the effects of strategic bombing and blockade would have forced Japan to surrender by the end of 1945 even if atomic bombs had not been used and the Soviet Union had remained neutral.