The answer to this is, of course, a matter of opinion. There is no objective way to determine what actions are and are not ethical. My own view is that the nature of terrorism does extend the range of actions that a country may morally and ethically engage in.
Terrorism is very different from organized warfare. Terrorism is carried out by people, like the 9/11 hijackers or the Charlie Hebdo attackers, who do not wear uniforms and cannot be easily identified as belligerents until they begin to carry out their attacks. Terrorism is intentionally carried out against essentially unsuspecting targets, not against military people who are aware that they are at war and liable to be attacked. In these ways, terrorism is morally different from conventional warfare.
Since terrorism is morally different from conventional warfare, we can at least argue that there is a different range of tactics that may be morally used to combat it. For example, in a conventional war, it would be immoral to carry out drone strikes against people who were not in uniform driving along in a civilian car far from the front lines of the conflict. By contrast, I would argue that it moral to attack people who have been identified as terrorists at any time and any place where there is a strong possibility of killing them without killing innocent bystanders. In a conventional war, it is not ethical or moral to try to coerce captured soldiers into giving information to their enemies. I do not believe that actual torture is ethical or moral even in the case of terrorism, but I do believe that it is permissible to be much more aggressive and coercive in trying to extract information from captured terrorists. (I freely admit that I do not know exactly where the line is to be drawn between aggressive and coercive questioning and torture.)
I would argue that different types of actions are moral in different situations. A situation in which a nation is faced with terrorist attacks is morally different from a situation in which the country is formally at war with a foreign country that uses conventional military tactics to prosecute the war. Therefore, a different range of options is moral and ethical when a country is seriously threatened by terrorists.