Terrorism is unconventional, asymmetric warfare. It is carried out by a small conventionally weak force that seeks to amplify its power by employing several characteristic techniques.
1) The terrorist attack takes place out of context and without warning. For instance, September 11 took place in a time of peace. Hiroshima took place in a time of war where awful things could reasonably be expected by citizens and the US issued a warning in advance to the Japanese government of its intention to use a terrible new weapon.
2) The attack takes place by non-uniformed people not in the regular armed services of any state who are disguised as non-combatants right up until the attack takes place.
3) It chooses targets nearly entirely because of their visceral psychological value. Defenseless civilians are an excellent target, for the natural sympathies of the populus can more easily be exploited. Military targets are not chosen for their military tactical or strategic value in any real sense, but only for the psychological value. Again, the intent is to amplify. On September 11, the attack on the Pentagon was meant to say, "We are godlike. We are powerful. We can hit you in your most inner sanctum." As a military operation, it could only be a failure. There is no way that a single plane could dent our defense department. The failed attack on presumably the White House or the Congress was the closest thing to targeting something of real strategic value, but the video of Bin Laden watching the attacks does not show him assessing any military aspects of the operation.
4) Terrorism is like a grotesque performance art. It employs gratuitous and spectacular theatrics to heighten the sense of dread. Again the purpose is to amplify the power of a small group. When assassins in a Lebanon hotel killed diplomats and knelt down and licked up their victims' blood, there was no military point whatsoever to that, but it increased the psychological effectiveness of the operation.
5) Terrorism seeks media attention to amplify. It really is a phenomenon born out of the media age. We would have bombed Hiroshima if not a single camera captured the image. In contrast, Al Quaeda designed the attacks on New York specifially for the cameras; that was the operation's raison d'etre.
6) Terrorism has apocalyptic and delusional overtones. From Bin Laden (jihad) to McVeigh (nationwide rebellion) to Manson (race war) to the PLO (pushing the Jews into the sea), the terrorist seeks to spark the final conflagration, whereas the Americans were seeking to end the conflagration in WWII.
6) Terrorism is nihilistic and eventually becomes the end in itself. The Americans bombed Japan because they were really trying to accomplish a reasonable strategic goal. For the terrorists and their like-minded fellows, however, seeing an American city get vaporised is its own reward. Think of the joy the Palestinians showed as the towers fell. They at that moment could not have known who had done it or what the strategic objective was, yet they knew that they heartily approved. That catharsis, visiting destruction upon the hated enemy and make that enemy feel pain for the sake of pain, is the true goal of the terrorist; all other considerations are secondary.
If so, then your premise is flawed from the start. In addtion, by your own standards WWII, and more specifically the nuking of Japan was total war. Another flawed measure. Total war disregards compromise. The essense of victory in total war is complete subjugation of the enemy.
By trying to support the bombing of Hiroshima and Nagasaki you've pulled the rug out of the greater good of WWII, which was the rebuilding of Japan and Germany. Again, not the ends but rather the means.
I hasten to add that your definitions of terror are westernized and fall short of the definition I've proposed. In all cases if the purpose is: warfare deliberately waged against civilians with the purpose of destroying their will to support either leaders or policies that the agents of such violence find objectionable.
Then the task is terrorism.