They were used on Japan and not Germany because they barely were okayed for use a few weeks prior to their detonation over enemy territory. The Germans had no compunction about dropping them on us had they had a working one so I don't think we would have had any issue deploying one over Germany --just like we had little issue firebombing Dresden. At the time saw both Japan and Germany as mortal enemies and would have used any resource at hand to defeat them militarily.
Actually, I think there is a big open issue about the difference between the US policy on strategic bombing in Japan and against Germany.
US bombing in Germany was designed to specifically attack industry. They flew during daytime, despite that leading to much higher causalities among their crews, and used bomb targets.
It was the UK that firebombed Dresden during night without bothering to aim at specific targets. The British always flew night raids and just aimed to cause carnage.
American policy on the Western campaign didn't target civilians. They used precision bombing (what it was called at the time, it wasn't very precise) to target real targets.
Yet, the policy against Japan ended up much different. At the beginning precision bombing was attempted. But by 1945 they started to resort to firebombing entire cities.
I wonder if there was a racial aspect to it. A lot of Americans consider themselves ethnically German.
I don't think there was a racial aspect in the sense that Germans were seen as more 'us'. We demonized both Germans and Japanese pretty similarly. There were Americans of German ancestry who changed their surnames to avoid discrimination and unpleasantness from other non German whites. I think we started firebombing Japan when it became difficult to tell industry apart from the general pop as they moved factories into non industrial areas
Still, I think, that Japan directly attacked the US at Pearl Harbor made a big difference. Germany was also seen as having a bad regime with war criminals, but they started the war with the US not with such a carnage. So, I think politicians and parts of the US people in this time saw Japan as the worse "beast".
Certainly we had people who thought that Japanese were more apt to side with our foe due to their being Japanese and being easy to tell apart. However as it pertains bombing, which is the discussion here, we were indiscriminate towards both. We did try to steer clear from cultural targets such as avoiding Kyoto. We did not have the same view on Dresden as it had more strategic importance. And people forget we had restrictions on Germans in the country and interned a few thousand as well though not to the extent we did Japanese. Italians on the other hand we didn't take seriously --not even sure we labeled them enemy aliens
Kyoto was the traditional seat of Imperial power. Tokyo, called Edo back in the days the seat of the Tokugawa Shogunate, a military dictatorship, is the national capital because of that. A whole lot of the action that overthrew the Shogunate happened in Kyoto, so sparing it was in part to encourage the Emperor---in a position that for a very long time had little or no real power---to do exactly what he did then. If he'd tried it earlier, he would have been killed and replaced with a more pliable one. As others have noted, the final surrender was a rather close run thing....
Hard to believe it was anything but European vs Asian. Not right, but it was war. And the British didn't do it like we did; and the British leaked secrets like a sieve. There was a highly-placed Japanese in UK govt, who sent everything on his desk back to Japan. So war is heck.
That's just rubbish. US bombing missions were not in any way targetted strikes compared to the UK's night raids. Have a look at what was achieved by the British pathfinder system guiding first Mosquitos then specialist heavy bombers to target mark the main raids.
The USAAF conducted firebombing of German cities, notably Hamburg.
