While I agree there's not a universally agreed upon definition of terrorism, I want to hear more about why you think bombing with novel munitions expressly delivered to dense urban areas which killed 250k people 90% of whom were civilian, in an attempt to scare them into surrender, is not "terrorism".
Killing people while trying to scare them into surrender is a feature shared by both terrorism and wars. A big bombing campaign done by an army is a war, not a terrorism.
Even when they specifically target civilians? What would you call a bomb exploded in times square? What if it were placed there by Iranian soldiers? Is that war, or terrorism?
If those soldiers are in full uniform bringing in an unconcealed bomb, and it's part of a broader campaign that's also going after military targets, I would not call that terrorism.
There is no distinct line between war and terrorism. Even before World War II, leaders would proudly and openly call for terrorism against their enemies and civilians. Most notably communist leaders like Lenin, who didn't shy away from using the word "terror" and "terrorism" to describe their own campaigns.
As for allied bombings, there is a chapter here on the term "Terror bombings":
The war was won by systematic, deliberate terror bombings, these weren't accidents or rare incidents.
From the great general Patton:
"We then went to the town hall and saw the Mayor, the Chief of Police, etc. I told Truscott to do the honors as he had captured Messina. The town is horribly destroyed – the worst I have seen. In one tunnel there were said to have been 5,000 civilians hiding for over a week. I do not believe that this indiscriminate bombing of towns is worth the ammunition, and it is unnecessarily cruel to civilians."
As for the Germans, they were among other things conducting terror attacks on civilan ships with their submarines, and openly calling their population to "total war".
The allies won the war by conducting terrorism of the largest level in human history. That might have been the only way for them to win. I don't think anybody here is a better general than Eisenhower or McArthur was, to be able to suggest a better way.
All sides of WWII were conducting outright terror attacks on the civilians of their enemies, with the allied terror bombings of German cities and terror nuclear bombings of Japanese cities being the most devastating of these.
