The "fear" part was achieved brilliantly by torching the ships and the walls and with a screeching Drogon towering above the people running for their lives. But if torching King's Landing was a message not to the people of the city (hard to have someone fear you if they're smoldering ash), but to the rest of Westeros, I agree with you that it makes sense, didn't see it that way.
My parallel with history was this...
History:
- USA has a weapon of mass destruction (atomic bombs)
- USA considers what to do to end the war (invasion of Japan maybe?)
- USA drops the bombs to force Japan to surrender, they do, and the war is over.
Game of Thrones:
- Dany has a weapon of mass destruction (the dragons)
- Dany considers what to do to end the war (avoid a bloodshed maybe?)
- Dany goes fo the nuclear option (using the dragon to target military objectives, like the fleet and the armored walls and the Red Keep where her enemy lives) and then...... carpet bombs the rest of King's Landing.
Feels overkill and useless just like for the USA would have been useless to continue to drop atomic bombs on Japan. But as I said - I didn't see it as a message to Westeros. If that's what it was, at least it makes some (terrible) sense.
In the spirit of discussion and debate, I think this is a fundamentally flawed use of the analogy primarily because you are equating the USA using its atom bombs to achieve what it wants (Japan surrenders, USA "wins" the war) with Dany getting what she wants when the Lannisters surrender. She didn't at all get what she wanted. She didn't care about the Lannister armies or Cersei or even ending the war, what she wants is to rule. Over the 2.5 episodes leading up to the bells ringing, she came to the realisation that defeating Cersei and "winning" the war wouldn't get her that because the people don't love her and won't accept her now that Jon has a better claim to the throne (whether he wants it or not). So Cersei surrendering isn't enough. To get what she wants she has to use her own atom bomb to destroy King's Landing and terrify the rest of Westeros into bending the knee. She won't have planned that in advance (I never thought she would and the showrunners have suggested as much), but that will have been the brutal logic that caused her to snap in the moment.
Again for the sake of discussion, I would re-work your comparison as:
History:
- USA has a weapon of mass destruction (atomic bombs)
- USA considers using its WMD to achieve what it wants (Japan surrenders, ending the war with USA victorious) but presumably struggled with the ethics of it and didn't immediately rush to use it (I don't know the detail of the history so happy to be corrected on this point).
- USA continues to battle without using its WMD but with no success.
- USA eventuaklly uses its WMD on two cities, indiscriminately killing innocent civilians, to force the rest of Japan to surrender, they do, and the war is over.
Game of Thrones:
- Dany has a weapon of mass destruction (the dragons)
- Dany considers using her WMD to achieve what she wants (the throne) but is talked out of it on a number of occasions.
- Dany therefore tries another approach - winning over the people of Westeros by helping them and getting them to help her, but events lead her to realise that this route cannot give her what she wants.
- Dany therefore uses her WMD on a city, indiscriminately killing innocent civilians, to try and force the rest of Westeros to accept her or risk meeting the same fate.