Taking into account history, isn't it justifiable to resort to terrorism in the face of a vast empire?

This is a really difficult question. If terrorism is the killing of civilians in order to achieve some political end, vast empires as well as fringe political groups commit terrorist acts--sometimes appealing to a state of war to justify their killings (but a declaration of war doesn't seem to make a moral difference.) If the history of terrorism by vast empires justifies terrorist acts by fringe political groups, then one would have to say that the violent measures taken by the empires (prolonged detention and torture of suspected terrorists, for example), are also justified. But if that is the case, what does 'justified' mean, and why should anyone care whether an act is 'justified'? One thought: 'justified' seems to mean at least two different things: the best thing to do, in the circumstances (which could be quite a bad thing to do, considered by itself), and 'a good thing to do' or 'the right thing to do', period. I think a terrorist act might be justified in the first sense, but not in the...