Follow The Money, page-7

The nuclear option was activated, first  because  they had the bomb a and, second because of the estimated costs to America in human lives of invading Japan.

"As American forces won territory ever nearer to Japan, military planners on both sides used casualty figures to construct their strategies. The Japanese intended to resist at all costs, deploying pilots as suicide missiles and expecting civilians to face invaders with sharpened bamboo sticks. Numerous Japanese citizens would be sacrificed, in this plan, to achieve better terms for peace.

On the U.S. side, President Harry Truman and his war advisers hotly discussed casualty estimates for a projected invasion. Congress and the public were solidly behind the war, but the president's advisers disagreed over the proposal's level of risk to human life, based on the number of Americans who had already been killed or wounded in the Pacific theater.''