President Truman decided to drop the first atomic bomb primarily for the following reasons:
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To destroy Japan's power to make war - The use of the atomic bomb was intended to force a swift end to the war by demonstrating overwhelming military power, thereby compelling Japan to surrender unconditionally and neutralizing its ability to continue fighting.
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To justify the expense of the Manhattan Project - The massive investment and resources put into the Manhattan Project warranted a demonstration of the bomb's effectiveness, which played a role in the decision to drop it.
The first point aligns closely with historical consensus regarding military strategy and goals during World War II, while the second point reflects considerations related to the investment made in developing the atomic bomb.