For the reasons stated above, to be sure. I would add that the method of attack, using hijacked planes full of involuntary victims as a suicide bomb, was designed to be high profile and to spread terror.
Hitting the World Trade Center was intentional, in that it panicked our financial markets and was instrumental in causing a recession. In other words, the fear they caused with these attacks hurt our economy, exactly as planned.
Al-Qaeda was not sponsored by any nation state, they were acting as an independent group with unconventional methods of fighting.
The attacks were designed to topple the WTC buildings (bin Laden is a civil engineer by training), and timed separately, so that one strike would look like a horrible accident, and the second strike would instantly spread the terrible knowledge that we were under attack.
If you look at the definition of terrorism that I gave you in a previous question, I think you can see this. There are a couple of reasons why this was terrorist and not military:
- The people who were killed had no direct link to any sort of military target. They were not soldiers, they were not engaged in producing military weapons, nothing.
- The attack was not truly in pursuit of any definite military objective. Al Qaeda was not trying to win a given war. Instead, they were mostly trying to punish us for what they see as our hatred of Islam and our actions which they see as opposed to their religion.
You can contrast this with the bombing of Hiroshima, for example, which was also not directed at a military target. But at least there we had a specific objective -- to make Japan surrender so that worse things would not happen to Americans and Japanese alike. This attack had no such objective. It was only meant to hurt us and make us afraid.