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.