Calling the bombings a war crime presupposes the legitimacy of limited war. The state of war is a socially accepted relation between person or group A and group B. It’s just that the moral rules — such as the international law — that specify the proper way of behaving given that relation were not followed, and so A’s atomic bombing B’s citizens was proscribed.
But what if we deny that war itself is legitimate? What if we affirm that the relation between A and B called “war” can no more be recognized than the relation called “slavery”? Again, we don’t have or try to work out the rules for proper treatment of slaves; why should there be rules for proper conduct of war?
Moreover, war is worse than slavery: a slave is at least alive and may have hope of escape. If we condemn slavery, then a fortiori, how much more war!
War is a way of life in (1) Stone Age autarky, (2) slave-owning society, and (3) feudalism, with each improvement of these social orders making war increasingly more limited. It was under feudalism that the law governing war-making came into its own and became best developed. The 20th century saw a reversion into total war, the consequences of which we are still dealing with. (4) Laissez-faire capitalism abolishes war as an institution.
It follows that if we do not consider war to be a legitimate state or relation between any two individuals or organizations, then the atomic bombing of the two cities can in no wise be called a “war crime.” Instead, under the ideology of capitalism, it is simply a crime, a mass murder of innocents of titanic proportions. The alleged war between “US” and “Japan” was nothing of the sort; instead, certain individuals, Smith, Jones, Robinson, etc. slaughtered each other for mysterious reasons and most unjustly.
At the same time, under the ideology of total war, everything is permissible, and there are no such things as crimes including war crimes.
Now slavery already presupposes the concept of property in things other than one’s body. The slaves, like merely material objects, such as weapons, forts, food, land, supply lines, etc., are tools of the slave-owner. Each slave-owner, in waging war against his peers, would want to conquer and plunder the enemy’s property. Moreover, such property, including the slaves, can be legitimately destroyed if doing so helps to secure victory. However, slavery is a step up from total war. A slave-owner would not want to cleanse an enemy slave-owner from the earth entirely. He wants the spoils of war for himself. Those include slaves. The result is that it is likely that rules will be developed to minimize property damage during a war that benefits no one. We thereby enter the realm of limited warfare, insofar as slaves are now somewhat protected from harm by law.
War becomes even more limited when we move up to feudalism in which workers graduate into tax-serfs. At this point, the tax lord’s peasants can no longer be considered solely his tools. True, they pay taxes to him, thereby financing his military campaigns. But they also live for themselves. They are partially free. They can even own property and run a business. The recognition of neutrality of these almost real human beings limits war still further.
Finally, capitalism does away with the legitimacy of war altogether. There are no proper rules for starting or waging war any more than rules for correct serial killing.