The Organa Connection
Before that, let me say that I agree that Dresden is an important apparent similarity. Dresden was an obviously important industrial and railway center for Nazi Germany, one that had largely been untouched by Allied bombings. (Hiroshima was chosen for the same reason.) Dresden had strategic value, so the massive bombing campaign against it wasn't intentionally to target civilians and break their morale. As a matter of explosives used, it was about the same as the bombings of other German cities, except that Dresden had the unfortunate circumstance of "perfect" weather conditions that exacerbated the firestorm. Regarding my moral views on war and targetting civilians, please read this entry, and scroll down to question 50.
In contrast, Alderaan was truly peaceful. Among its residents were active Rebels and their sympathizers, but the planet itself was not a locus for the Rebellion; at least, not a significant one. Alderaan was not an evident industrial center, unlike Dresden was for Nazi Germany, or even a hub of Rebel travel. The Empire would have easily detected a Rebel base on a major planet like Alderaan; if anything, combat ships coming and going are a giveaway. Alderaan may have been a secret communications hub, but the Empire knew that the real Rebel base must be elsewhere; otherwise Moff Tarkin would have simply destroyed Alderaan as a suspected Rebel stronghold. It would be comparable to nuking Riyadh to strike against the insurgents in Iraq (like the 9/11 terrorists, most are Saudi). Like the Empire destroying Alderaan, that would do nothing to attack the heart of enemy forces, but it would kill a great many people who are innocent and uninvolved.
Perhaps Leia's father, Bail Organa, was a military target? He was on Alderaan, after all, and apparently he was the only other person who could retrieve the Death Star data from R2D2. But the Empire clearly did not know of his involvement, let alone that Leia intended the plans for him. It would have been effortless to arrest and torture Organa on the slightest suspicion. Tarkin and Vader would have also used Organa to coerce Leia into confessing everything, before threatening to destroy Alderaan. Moreover, consider that Leia would have never risked sending the Death Star plans to her father, if it was even slightly possible that he was under Empire suspicion and surveillance.
Maybe Organa, by virtue of his daughter, was finally suspected of being part of the Rebellion, and perhaps the Empire really was afraid that arresting him would increase support for the Rebellion. Or the Empire didn't arrest him, hoping that surveillance would yield more benefits than his execution. Still, the timeframe makes this unlikely. Only a few days, possibly a week, elapsed between Leia's capture (at the very beginning of Episode IV) and Alderaan's destruction. It would have been elementary for the Empire to arrest Organa the moment that Leia was seized. Instead, after a few days, the Death Star wasted time and energy to travel to and obliterate an entire planet, which wasn't even home to the enemy base. It's not a very efficient way of "crushing the Rebellion with one swift stroke."
However, Tarkin himself made the Death Star's purpose clear:
Tarkin: Princess Leia. Before your execution, I would like you to be my guest at a ceremony that will make this battle station operational. No star system will dare oppose the emperor now.(emphasis added)
Leia: The more you tighten your grip, Tarkin, the more star systems will slip through your fingers.
Tarkin: Not after we demonstrate the power of this station. In a way, you have determined the choice of the planet that is to be destroyed first. Since you are reluctant to provide us with the location of the Rebel base, I have chosen to test this station's destructive power on your home planet of Alderaan.
Leia: No! Alderaan is peaceful. We have no weapons. You can't possibly...
Tarkin: You would prefer another target, a military target? Then, name the system! I grow tired of asking this, so it will be the last time. Where is the Rebel base?
Leia: Dantooine. They’re on Dantooine.
Tarkin: There, you see, Lord Vader? She can be reasonable. Continue with the operation. You may fire when ready.
Tarkin: You're far too trusting. Dantooine is too remote to make an effective demonstration, but don't worry. We shall deal with your Rebel friends soon enough.
Tarkin accepted Leia's answer. Therefore, his decision to destroy Alderaan anyway was an act of government terrorism. Eliminating the Rebel base was important, but so was demonstrating the Death Star's power. Recall the earlier scene:
Tarkin: The Imperial Senate will no longer be of any concern to us. I have just received word that the Emperor has dissolved the council permanently. The last remnants of the Old Republic have been swept away.It's true that Leia did lie several times, but any good spy does. But why must that mean everything she says is a lie? In fact, by the time she meets Tarkin on the Death Star, her involvement with the Rebellion is quite obvious. She no longer denied it and instead remained silent.
Commander: But that's impossible. How will the Emperor maintain control without the bureaucracy?
Governor Tarkin: The regional governors now have direct control over their territories. Fear will keep the local systems in line -- fear of this battle station.
A thought on treason. In my previous post, I asked, "Against a corrupt, evil government, can there be such a thing as treason?" Yes, "from a certain point of view": if you're the corrupt, evil government. In an absolute sense, however, no.