Answer 2 atomic weapons.
They could not have dropped another one. The second might have been avoided had the Japanese surrendered more clearly and quickly. It is an odd quirk of history that Eisenhower opposed both of them because he thought both were unnecessary. The Japanese were militarily finished at that point.
His boss, Henry Stimpson [then Secretary of War], according to Eisenhower's biography, chewed him out for expressing such an opinion.