It is justifiable for any country to start a war based on a terrorist act if that crime can be proven that it was aided by a country or said country is harboring any wanted suspects. I do not think that just because a terrorist comes from a certain country though justifies the whole nation to suffer the consequences.
It would be justifiable to start a war over an act of terrorism that was a major act resulting in massive damage and casualties, such as the Twin Towers on 9/11. It would almost be necessary to declare war, otherwise the country would be viewed as weak and unable to defend itself. You cannot declare war over every act or terrorism, but for one the magnitude of the Twin Towers, it would be reasonable that it would start a war if the offending nation could be identified.
The mob mentality equates not just to your own mob, but people want to identify others as belonging to a mob. The idea that a small group of extremists could inflict such harm is beyond what people want to admit. Six guys couldn't do that much damage. There must be a country behind them. There must be some conspiracy behind it all that allowed so few to kill so many. The truth is, terrorists represent other terrorists and thats it. There is rarely a country behind them.
An act of terror is not a declaration of war. An act of terror should be condemned for sure, but also investigated to ascertain the motivation. Acts of terror are not sanctioned by countries, but are acts committed by people, or groups of people, rarely associated with the average citizen. It would be wrong to engage in war against a population when a small group commits an act of terror.
Terrorism is, by definition, about being grandiose and galvanizing attention. To declare war over an act of terror usually is to play directly into the hands of the people who committed the act. It legitimizes their attack and shows that they were a force to be reckoned with, and further, that the resources of a nation should be set against those like them. Given the rather low probability of dying in a terrorist attack - you're more likely to die in a crashing airplane - it makes little sense to spread war over a single act.