> The US has had plenty of moral lapses in its military adventures, but one suspects that the powers of the past 500 years would have been far more willing to obliterate entire nations without concern for civilians.

Simply not true. There was a reason why european wars were fought in the fields and not in cities/population centers in the past. Also, past powers tended to conquer nations/cities in order to subjugate the population for exploitation. After all, humans were valuable resources back then. It is very rare that a military/government/nation would simply choose to exterminate the inhabitants of a continent ( 100 or so native nations that were wiped out ) or set entire cities full of civilians ablaze ( dresden, tokyo ) and nuke two civilian cities ( hiroshima, nagasaki ).

Pretty much the US and our side kick britain are the only ones to be guilty of the first two. We are still the only ones guilty of the 3rd.

But precedents being set, your assertion is likely to be truer in the future. I suspect that in the next 500 years, powers would be more willing to obliterate entire nations without concern for civilians, especially if technology and automation make humans a less valuable commodity.