"Use of the bomb ironically was an act of mercy"
That's the official propaganda, put forth by the only country to ever have used atomic arms. The old "we saved a million Americans and 10 million Japanese" rhetoric is quite old and threadbare, given the evidence of Japan's (and America's) true state in mid 1945.
The bomb was used as a means with which to terrorize Japan (as if firebombing and carpet bombing weren't enough) into surrender. Without the demonstration of "total destruction" as laid out in the Potsdam Declaration, Japan would have continued to fight to the last man. Japan, realistically, in all probability, would have starved by early 1946 (this is according to a United States Naval Intelligence report issued in April of 1944, long since backed up by actual Japanese records), before any American invasion would have been feasible in the first place.
If necessary, Truman would have suffered that mystical "million casualty" number, if only to prop up the ridiculous notion of a united allied front (a premise which had far outlived it's usefulness) and the nonsense of "unconditional surrender" as laid out in the Potsdam declaration and it's forerunners (Tehran conference, Atlantic Charter, Cairo Conference, etc. Japan's surrender was certainly NOT unconditional, despite the allied propensity to pretend it was).
Military art being what it was in 1945, Truman could not end the war, conventionally, in any other way with the existing resources at hand, and the political realities back home. The tactics of naval blockade (the strategy favored by Nimitz), and siege would only have accelerated the process, but it's possible that Japan would have surrendered, or fallen to internal revolt, long before an allied soldier set foot on the Home Islands (the strategy favored by MacArthur).
The bomb was an expedient. The attempts to describe it as an instrument of saving lives do not change that reality, because the only lives being saved were AMERICAN (attempts to rationalize the killing of 150,000+ Japanese in this regard, were always an afterthought), and they were being "saved" in this fashion because the American public had tired of war, had sacrificed enough on the altar of war, and had simply run out of soldiers to be used by an increasingly frustrated and uncreative military leadership. We could not have "won" (at a price worth paying) by most other means, and waiting for Japan to rot from the inside out was beyond our patience or political will at the time.
Now, if you call the mass murder of innocents that resulted from the atomic bombings of Hiroshina and Nagasaki, undertaken to prove an ultimately worthless political point, in support of a war effort that was running out of steam and materiel, and laboring under the weight of adverse public opinion, on behalf of a frustrated military and diplomatic complex to be "humane", then you have a strange definition of "humane".
Every weapon is an "expedient" and all are designed to end the conflict as soon as possible in order to save lives. That is exactly what happened wrt Japan. There appears to be no issue here except to those who want to limit America's use of its power.
Propaganda is not necessarily false in any case and it is not in this one.
The Japanese were starving on Saipan and Tinian and Okinawa prior to our arrival. They still fought to the last man, even committing suicide to avoid capture.
Let there be no doubt, indubitably a mainland Japan assault would have taken a million US casualties. Additionally, once warfare spread by small units into the mainland, family resentments against anything American would have lingered with force for generations,..far more considerably than anything perceived today. Those estimates were not historical revisionism, they were logistical estimates based upon professional planning rigor developed over the 3 year island campaign, rehearsed and finetuned with uncompromising dedication in a life and death struggle of total war. Those making the assessments were PhDs in uniform with degrees and professions in history, accounting and logistical professions. The best men of the nation were in place making those assessments with a strong historical experience in validating and preparing rigorous plans.
Recall, that Japanese soldiers were still being found on islands in the Pacific 40 years after the war had ended who still considered themselves at war.
BTW, civilians have always suffered the largest percentage of fatalities and casuaties from warfare. That is why refugees leave their homes. It is also why on an island nation such as Japan, the one million casualty estimate had also been appraised as possibly being considerably lower than past datum might have concluded. The Japanese on Okinawa didn't surrender, nor did they anticipate reinforcments.