The words “needed to force” in the question implies that the atomic bombing of Hiroshima and Nagasaki was the only measure left while the words “surrender unconditionally” means that there would be no guarantees made for the country that surrenders. In order to put an end to the horrifying war which took millions of lives, the bombing was indeed necessary. Only the presence of an overwhelming demonstration of destructive capability would serve as a shock to and be sufficient in changing the mindset of the stubborn Japanese military, making them surrender unconditionally.
However, traditional firing of air raids would have been another alternative even though it would require a longer time and greater efforts. Thus, I would argue that only the atomic bombing of Hiroshima was needed to force the Japanese to surrender unconditionally. The atomic bombing of Hiroshima and Nagasaki was the only measure left to make the Japanese surrender without any guarantee in order to prevent greater loss of lives. The war had already resulted in millions of deaths and a continuation to the war would only cost more lives.
To prevent the number of casualties from rising, it was necessary to force the Japanese to surrender. The Americans were left with the choice to either invade Japan or to drop the atomic bombs to force Japan to surrender. However, it was estimated that the invasion of Honshu and Kyushu would cost 1 million casualties to American forces alone whereas dropping the atomic bombs would only result in casualties in Japan and was thus seen as a better sacrifice for the war to end. Moreover, the effect of the bomb was a better promising solution to end the war due to its destructive effect.
Thus, the only measure left to prevent the number of casualties from increasing was to drop the atomic bombs to create a destructive effect in Hiroshima and Nagasaki so that Japan would be left with no other choice but to surrender. However, Japan was already running out on war resources due to the embargoes. Japan was also beginning to suffer as Allied aircraft and submarines began to cut the shipping routes that brought raw materials and food into Japan from her territories in Asia Pacific. Japan was running out on oil and petroleum, which are crucial for war efforts.
Even though oil fields in the East Indies were under Japanese control, the sea routes in which the ships had to sail pass in order to import the oil and petroleum to Japan were subjected to constant attack from the Allies. Without these resources, it is impossible for Japan to continue the war. Thus, Japan would have surrendered after a period of time even if the bombs were not dropped. However, without the dropping of the bombs, it would have been impossible to make Japan surrender without any guarantees. To the Japanese, surrender would be a threat to the Emperor.
Thus, Japan would only agree to surrender if guarantees were given that the Emperor would be allowed to retain his throne. Thus, the atomic bombing was needed and the only measure left to force the Japanese to surrender without any guarantees. Yet another reason as to why the atomic bombing of Hiroshima and Nagasaki was the only measure left to make the Japanese surrender without any guarantee was because of the stubborn Japanese military. The Japanese military had the mindset that “A true soldier would rather die than to surrender”.
Thus, it was believed that the Japanese had intended to fight to their deaths. Even after the Americans modified the ultimatum and demanded only an unconditional surrender from the Armed forces of Japan after they discovered that an unconditional surrender would be seen as a threat to the Japanese Emperor, they refused to surrender. Ironically, the softening of the surrender backfired and instead, the Japanese saw it as a promising development from the enemy and assumed that the Americans no longer insisted on a full unconditional surrender as circumstances have arisen that forced them to end the war.
Prime Minister Suzuki thought that by insisting on their demands, the Americans would eventually give in to their requests. Hence, Japan stubbornly refused to surrender. Thus, the only measure left to force the Japanese to surrender without any guarantees was to drop the atomic bombs, which would result in destructive effects to shock the Japanese military into surrendering. However, not the entire of Japan was against surrendering. Unlike Germany, Japan was not ruled by a dictator but a system of government which includes a prime minister, a cabinet made up of military, civilians and a parliament.
After the Potsdam Conference in Berlin on 16 July, Prime Minister Suzuki and foreign minister Togo were considering a negotiated settlement. Even the Emperor gave instructions to Prime Minister Suzuki to find a way to end the war as soon as possible. Japan also started to send peace offers to the United States, showing her desire to negotiate for surrender. However, it was also clear that these were not enough to end the war. The Japanese military and General Korechika Anami were determined to continue the war.
Soon, a political stalemate was present between the civilian leaders and military leaders, where the civilian leaders tried to find ways to negotiate to an end of the war whereas the military leaders wanted to continue even if they had to place their lives at stake. As Japan could only legally enter into a peace agreement when unanimous support was achieved in the cabinet, orders were given that Japan would ignore the revised ultimatum. Hence, only the dropping of the atomic bombs would make the Japanese military realise that they should surrender to minimize further casualties.
Thus, the atomic bombing was needed and the only measure left to force the Japanese to surrender without any guarantees. On the other hand, the atomic bombing of Hiroshima and Nagasaki was not the only measure left to make the Japanese surrender without any guarantee because firing fierce conventional air raids would also be an alternative. All along, the Americans have been firing constant air raids, which have caused considerable damage, killing about 241,000 to 900,000 Japanese.
The air raids have also resulted in extensive damage to many cities in Japan as well as a huge decline in industrial production. Hence, firing constant conventional air raids would have been able to exert sufficient pressure on Japan to agreeing to an unconditional surrender without releasing the atomic bombs. Thus, the atomic bombing was not needed to force the Japanese to surrender without any guarantees. However, firing air raids would require a longer time to force the Japanese to surrender without any guarantees.
Meanwhile, there would also be more resulting casualties. On the contrary, the atomic bombs would have a faster effect on Japan as she would only be left with the choice to surrender, fearing that America would drop atomic bombs in other parts of Japan and eventually mark the end of the Japanese empire. Thus, the atomic bombing was needed to force the Japanese to surrender without any guarantees. After reconsidering the above points, it is conclusive that the atomic bombing was indeed needed to force the Japanese to surrender unconditionally.
However, the atomic bombing on Hiroshima alone would be sufficient. The duration between the two atomic bombings was three days, which was insufficient for Japan to realise the destructive effect of the nuclear bombs. In addition, Japan was already planning to surrender previously. Hence, if Japan was given more time to understand the effect of the nuclear weapon on her country and people, she would have surrendered unconditionally. Thus, only the atomic bombing on Hiroshima was needed to force the Japanese to surrender unconditionally.