This is a VERY difficult question. I am really really against violence, but sometimes you got no choice left. Does the invasion give you the right of killing people, whom themselves mostly don't even support the case (e.g. many soldiers in nazi-germany didn't know what they were doing, for they were much too blinded by the illusion Hitler had created) I'll think about this some more and then fill in the poll...
I can't pick either choice because it really depends on re kind of invasion. In seriousness, if I was aware of and certain the invasion was hostile towards my country and not say: an invasion of people escaping a different war, then I would go to war and fight to preserve our freedom and any other countries freedom that was also being oppressed by said invading country.
Now, in a less likely scenario: say we were being invaded by aliens. Well, then I would want to know if they are hostile are not. It all depends on circumstance really. But usually I would want to fight to preserve freedom and fight for the right reason.
By earthly values, it would make sense to go to war in order to progress freedom from those that oppress and kill (thus a just war). But I'm no judge to execute who should live, who should die, and who is more valuable as a free-willed living sentient being, so no I wouldn't go to war myself, no matter how selfish it might seem to those that would otherwise innocent who would be killed for my not protecting them.
What I'm saying is not rational though. I'm a person based on faith, hope, and love, not one based on reason, natural actions, or justification. Though I definitely am not immune to it. My mind would think differently from my soul, and I would probably fire a gun at a person who is killing those right in front of me (including myself). I'd be against it fundamentally, but I am weak willed, and would probably fulfill those natural thoughts of self-defense. I'm only human after all. There is no right answer to a problem such as this, where the world can make war and disagreement of who should live and die.