Terrorism is one of thost pop phrases that have been used so often it has lot alot of impact, other than to denigrate an action with a label. Originally, it was used to apply to certain acts of violence which were desgined to draw media attention to a particular cause or group. Since then, its defenitions have broadened to include any form of violence for which people don't agree with. I am not saying that horrible actions dont occur regularly, and that they may inspire terror, but I think the use of the term terrorism is a worn out. Its definitions are so broad that it could be applied to any form of violence. I would define terrorism as a label people use to simplify and ostrize various forms of violence.
As for your original statement
Following this logic, the US responded to a military attack with one of the greatest acts of terrorism in history.
First, you imply that the use of nuclear weapons was based only on the attack of Pearl Harbor, and glossed over the several years of continual combat between the two nations. A gross oversimplification. Secondly, I would argue that WW2, as a whole, constitued the most bloody, least ethical (if you can find any ehtical war), and most destructive event in human history. I am not arguing the point of the ethical use of the atomic weapon any more than I would argue the ethical use of any means of mass murder. War inherentaly is about the destruction of life and property, and has always claimed far more innocent lives than than the lives of soldiers. In context, your opinion that the atomic bombs represented some special unique horror is absurd. The means of death may have been novel, but the results and scale were no worse than many of the atrocities of ww2 by all sides.
As for "deserving" to be bombed, in war, the goal of both sides is to subjegate the enemy by any an all means. It has nothing to do with one or another nation "deserving" to be attacked. Heck, there are many people in the world who thing the US desrved to be attacked like we were in 9/11. Agreement or disagreement with that sentiment doesn't change the end result of what happened. I guess I simply disagree with your particular view that the use of the bomb was a special atrocity any more than the firebombing of dresden, or the german v2 attacks on britain, etc. I hold that all war is based on atrocity.
In regards to the use of the atomic weapons, their use was tragic, but as I stated previously, hardly unique other than the novelty of their method of destruction.
I do not believe that the furthering of military goals should include the murdering of 'enemy' civilians, regardless of the atrocities committed by one's enemies. When such acts are committed, they are atrocities in themselves. Thus I also would never support the wholesale destruction of any city, since the civilian casualties are far beyond unavoidable collateral damage.
While I might agree with your personal view in this regards, our personal opinions hardly change the circumstance of previous wars. There are great many things I find atrocious in history, outside of war, which were deemed acceptable at the time. The best we can do is understand the context of that time, learn from the mistakes made, and try to not repeat the same mistakes in the future.