In the Second World War, the UK began a bombing campaign against German cities, with the aim of damaging the 'morale of the enemy civil population'. The US later joined in the bombing campaign (though maybe not with the same explicit aims?), and began its own bombing campaign in Japan. Firebombing of Tokyo killed up to 100,000 people, and the two atomic bombs killed over 150,000.
It seems - and correct me if I'm wrong - that attacking civilian populations like this was deemed acceptable by the US and UK, even if it remained controversial.
Today we live in a world where this sort of indiscriminate attack on civilian targets is usually judged unacceptable both morally and legally. The 1949 Geneva convention put in place some somewhat vaguely defined(?) protections for civilians. The 1977 addition makes these protections more specific, prohibiting "indiscriminate" attacks, and giving some detail on what this means.
So I have two related questions.
The broader one is: how have we got from there to here? When did Western militaries accept and start teaching that this was unacceptable? What resistance has there been to the changes? How was the discord between this norm and the doctrines of nuclear war managed?
The more specific question is: what instances after 1945 are there of Western militaries attacking civilian targets with the explicit or implicit aim of coercing the civilian population?
The more specific question is: what instances after 1945 are there of Western militaries attacking civilian targets with the explicit or implicit aim of coercing the civilian population?
Well, for one, the Korean War which began in 1950 saw the US use B-29 bombers to bomb Pyongyang and other cities in North Korea. However, as I'll explain later, the deliberate targeting civilians already begun to fell away as a strategy.
The broader one is: how have we got from there to here? When did Western militaries accept and start teaching that this was unacceptable? What resistance has there been to the changes?
Studies after WW2 were conducted about the effectiveness of strategic bombing. It found that the deliberate targeting on civilian centers wasn't all that effective in terms of breaking the civilian morale. Not only did strategic bombing of German cities day and night not end the war any sooner (the Soviets taking Berlin and the Allies closing in from the west prompted their surrender), but the Allies suffered their own forms of bombing (England during the Blitz) and instead found their citizens more resolute in defending their own homes.
Where strategic bombing did become more useful was when it was targeted at infrastructure and other assets that their military would use. Bridges, rail centers, transportation hubs, etc. You see this in Korea where B-29s started dropping radio bombs to target bridges/railways, and in Vietnam even our major bombing campaigns with strategic bombers like the B-52 (e.g. Operation Linebacker II) were targeted at ports, railways, supply depots, etc.
And finally, beyond the fact that military studies finding deliberate targeting of civilian centers being less effective, there was the fact that warfare had changed.
Today it takes a single B-52 with a crew of 5 to drop the same amount of bombs that 16 B-17s with 160 total crew members took from London to Berlin. And oh yeah, that B-52 took off from Louisiana.
Because of this, we employ far fewer bombers - which also makes each bomber significantly more valuable. Losing a B-52 much less a B-2 today would be extremely costly - instead, we employ our bombers completely differently. We found, during Vietnam, that bombers like the B-52 could be very vulnerable to surface to air missile systems.
Hence, in the post-Vietnam era, the USAF focused on fast bombers that either flew extremely high (like the XB-70) or low (like the B-1) and then on stealth aircraft (the F-117 and then of course the B-2). The B-52 has become more tailored to carrying long range cruise missiles as a stand-off missile platform (though it can carpet bomb as it used to as well).
So to answer your question: the studies done after WW2 and the lessons learned in Korea and Vietnam have changed military doctrine regarding aerial bombardment. Not only that, but changes in air defense and in bombing technology have more or less ended the days where bombers fly in massive formations to indiscriminately carpet bomb large areas.
How was the discord between this norm and the doctrines of nuclear war managed?
Nuclear war has been treated as a separate entity from strategic bombing really ever since the Soviet Union developed their own nuclear weapons and the capability to deliver them.
The idea of mutually assured destruction has more or less relegated nuclear weapons into two categories: tactical and strategic, with strategic being more aligned with the idea of wiping out an entire civilization.
Both sides drew up numerous use cases for nuclear weapons. Some believed that tactical exchanges against enemy armored formations would be acceptable - indeed, it was suggested that if the enemy used nuclear weapons strictly on military targets only, we'd respond in kind.
The whole idea that "if one nuke goes off, we wipe them completely out" is a misconception a lot of people have about nuclear weapons. All that stuff is way above public discourse for obvious reasons, but using nuclear weapons to wipe out an entire country's populace is not a frequent reason for the use of nuclear weapons.