Because Hiroshima and Nagasaki were real events, because they happened, there can be no gainsaying their horror. Operation Downfall did not happen, so there’s a lot of gainsaying. Would the Japanese have been awed into capitulation by an offshore A-bomb test? Did the Soviet Union’s invasion of Manchuria, starting the day of the Nagasaki bombing, have the more decisive effect in pushing Japan to give up? Would casualties from an invasion really have exceeded the overall toll—by some estimates approaching 250,000—of the two bombs?
We’ll never know. We only know that the U.S. lost 14,000 men merely to take Okinawa in 82 days of fighting. We only know that, because Japan surrendered, the order to execute thousands of POWs in the event of an invasion of the home islands was never implemented. We only know that, in the last weeks of a war Japan had supposedly already lost, the Allies were sustaining casualties at a rate of 7,000 a week.
We also know that the Japanese army fought nearly to the last man to defend Okinawa, and hundreds of civilians chose suicide over capture. Do we know for a certainty that the Japanese would have fought less ferociously to defend the main islands? We can never know for a certainty.