By definition, sure, you could say that America committed terrorist acts in WW2.
If "terrorism" is defined as the intentional targeting of civilians: It'd be tough to argue that dropping the a-bombs on those cities didn't qualify!
I think most people would say those "terrorist acts" were justified as necessary measures to prevent Hitler from taking over the world.
I remind you: We're not discussing when terrorism, or war in general, is just or necessary! You can have the most just cause in the world. But if terrorism is the intentional targeting of civilians, and if you target civilians: You are a terrorist! The morality of being a terrorist is another matter.
But linguistically speaking you're only a "terrorist" if you're using violence for political gain:
By that definition: All soldiers are terrorists! When two countries go to war: They're both using violence for political gain, to defend their political order. So, that's not a practical definition if "terrorist" is to be a useful term.
Terrorist has come to mean the random bad guys that kill people and blow stuff up without regard or remorse, including themselves. I don't personally see a problem with that.
How do you know that some soldier (be it regular, irregular, insurgent, or otherwise) who's blowing something up has no remorse or regard? That definition relies too much on reading the soldier's mind, it's not practical if "terrorist" is to be a useful term. Unless the point of the term is to be propaganda, to make us assume that the enemy soldiers are evil people who just love killing.
But on that note: I would hope that a soldier would see the deaths he causes (civilian and military deaths alike) as cruel necessities of war, and not as positive goods. Of course, there are bad people in the world, and soldiers are no exception. There are soldiers (again, including irregulars, insurgents, etc.) who just like killing people, and have no regard for human life.