War

Is it possible for a war to be fought in which both sides are justified? Or is every war necessarily problematic in the sense that at least one party must be wrong?

The short answer I think is, yes, one could imagine situations where all the participants in a war had strong reasons for participating. A more insightful answer to your question, however, will depend on exactly how one understands the ethics of war and peace. For example, if it turns out that pacifism is correct (for more on this doctrine, see http://plato.stanford.edu/entries/pacifism ), then the answer to your question may be--depending on the exact version of pacifism that is true--"no" because it turns out to be impossible for any act of warfare to be justified. On the other hand, if just war theory is correct (for this, see http://plato.stanford.edu/entries/war/#2 ), then one can imagine situations where multiple parties to a war were justified according to the rules of jus ad bellum. Since those rules are pretty strict, however, in practice I imagine that this rarely occurs, if it ever does. (It is possible, however, under this doctrine -- it is not the case that "every war is...
War

I have an opinion I'd like some feedback on. My view on war is generally that it's a bad idea. Aggression against another country or similar entity is difficult to justify. However the fact remains that an outside force can invade and make war on your country. My opinion on this is that an invader should be destroyed completely. Ruthless exploitation of any weakness, and use of any weapon is completely justified to expel the threat, at least until they have ceased their aggression and given back any territory gained. After that it would be difficult again to justify continuing the use of ruthless tactics in an act of aggression towards your enemy in their own territory. My idea of using complete force against an aggressor comes from that you didn't make war on them. They brought war to you. For example, if you were being violently mugged, it would be justified to kill your assailant. However, it would be unjustified to go out and kill someone just because they might mug you. Or, if you were mugged and...

I agree with the thought that being the subject of aggression does not necessarily license extremely violent responses like killing, and I would add that pacifists believe there can be--depending on the exact pacifist views being considered--principled and/or pragmatic reasons for refusing to respond to aggression with any form of violence directed toward the aggressor. So, for example, Gandhi believed that a pointed refusal to respond to aggression with violence against the aggressor could serve to change the behavior and attitudes of the aggressor and of other witnesses to the aggression. A useful summary of some assessments of pacifism by philosophers is here .
War

How does one respond to the argument that the danger to our troops is misguided because a citizen of Washington D.C. has a greater chance of being fatally shot than an American soldier does of dying in Iraq?

That there are problems in Washington DC by no means undermines our reasons for caring about the Iraq war and its dangers. With respect to caring about people in danger: Shouldn't we have concern for the welfare of those involved in the Iraq war (Americans and others) as well as those who live in areas with high rates of violent claim (U.S. citizens and others in Washington DC and elsewhere)? Likewise, with respect to discussions about public policy I think both these problems are important and worthy of discussion and debate.