Prior to the Pearl Harbor attack on US materiel parked in the Hawaiian colony, B2 bombers were being produced for the US armada. There was a national narrative in the US: they would be used to "burn Japanese paper cities to the ground."
And that is precisely what the US did.
Besides the historic imperial city Kyoto, all population centers in Japan larger than that of Hiroshima/Nagasaki had been destroyed in US bombings, bombings that themselves amounted to more destruction than the atomic weapons.
I don't use the term destruction lightly. Often, these were largely paper and wood cities, after all, and firebombs were specifically implemented in order to destroy as many civilian buildings and residents as possible.
If they had been American cities of the same size, it would look like this:
Cleveland Firebombed: 58% destroyed
New York Firebombed: 51% destroyed
Los Angeles Firebombed: 40% destroyed
Chicago Firebombed: 35.1% destroyed
San Diego Firebombed: 37.6% destroyed
Baltimore Firebombed: 55.7% destroyed
Miami Firebombed: 35.8% destroyed
Richmond Firebombed: 63.4% destroyed
Nashville Firebombed: 41.4% destroyed
...58 more cities, then Hiroshima and Nagasaki.
The tactic was mass human death. The same tactic was later used against Korea, Vietnam and Laos. The preeminent power in the region was Japan, and US policy for the region was to replace them with the US as hegemon, and Europe as secondary power. Japan was to be a subservient 3rd power to the US and Europe in the region, thereby securing US regional influence comprehensively.
In order to effect the level of desperation needed to control a new vassal, the US needed to teach the Japanese across their nation that they were under threat of death from US munitions at any time, and now that your cities are destroyed, let US capital expand to rebuild them. Japan was a fresh market, long the target of aggression from the US and Europe to open to US trade in an exploitable "free market" which would be dominated by the established, unfree markets in the US and Europe.
A Japan with deleted infrastructure, a decimated population and complete political capitulation was perfect for such market expansion.
The US had already wasted enormous productive capacity on destroying the cities. It was an investment impossible to neglect. It was also a sacrifice of human life meant to consummate the hegemonic, political and capital investment of the US.
The targeting of civilians was specific. These were political bombings meant to effect political change via the mass killings of civilians, meant to effect terror and acquiescence. In other words, the bombing of Hiroshima is the definitive form of "terrorism" as defined in official US doctrine.
It is a war crime to target civilians and civilian infrastructure; military targets are legitimate.
The US is the single most important potential target for International Criminal Court (ICC / Rome Statute) enforcement. It has the overwhelming power across the world, and it bears responsibility for terror far more than any other power today.
The principles used to bomb these and every other target of US aggression would result in punishment of insanely larger magnitude when applied to the US for its share of threats against the world. Quite literally, the entire US nation would be emptied if its foreign victims had equivalent revenge.
Revenge (for simply defying the US) is the very core of US justification for its corrective military engagements. Of course this hegemonic policy is illegitimate, criminal and incredibly destabilizing. Likewise, The WWII Bombings of Japan were illegal and immoral.
What We Should Have Done
Japan, a nation which was willing to negotiate (and thereby legitimize) a surrender, had the right to protect its citizens from an onerous occupation or any other stripping away of the human rights of its citizenry. If a criminal has rights under a just and legitimate legal system, a nation with millions of people with varying levels of responsibility have quite many more rights.
The right a nation or people has is to reason. Reason based on the capacity of our legal, moral and productive technology and advancement. Reasonable standards of culpability, resolution of conflict, restoration of rights and sustainable, legitimate institutions.
This is the only legitimate policy of democratic, pluralist systems. It is also highly sustainable, as it promotes the cooperation between populations based on legitimized authority, a shared standard with formal methods which can be audited, and the potential to eliminate political and economic relationships which provide exploitable populations for dominant powers.
Despite this, such comprehensive legitimacy and nuance was not needed to save the lives of hundreds of thousands of Japanese, and their cities. The US need simply to have agreed to a legitimate settlement negotiated with the rights of Japanese given rudimentary consideration at least.
The US could have stalled to enrich their position as they starved the Japanese economy and contained them. Loss of life is readily kept at minimum as actual threats to embargoes and occupations are dealt with piecemeal, as they usually are at the nadir of counterinsurgency style conflicts. The US only has to display a modicum of sanity to have protected human life quite comprehensively in its Japan strategy.
That is to say, the US need simply treat innocent human life as worthy of living. But the US chooses another form of policy: the legal doctrine that there is human life unworthy of life. It is this doctrine which precludes US deference to the ICC, or indeed any lawful activity impacting other sovereigns. Iraq somewhat recently requested this consideration - that US military eventually become responsible for acting within the law of the land in Iraq. This demand prompted Obama's pullout, as US sovereignty is considered the only legitimate sovereign in such a monopoly-power system.
The American State cannot be trusted with the power it possesses. That is true for the principles of legitimacy, democracy and stability, which are deeply undermined under a US hegemon. Much of US power should pass to international institutions, and much of the power over other states should never be accumulated in the first place.
http://www.ditext.com/japan/napalm.html (city listing)