Unfortunately, I know too many veterans from various wars and police officers ... some close friends and family ... who can share like tales of being involved in innocents, children, killed in the heat of combat. They all carry scars, and will do so forever. Even those who believe it was unavoidable feel the weight. Such is the weight of Karma.
I long ... I foresee ... a world in which our capability to do violence against each other has been evolved past. There will be a world without wars and violence.
But for now, I do feel that some wars ... although tragic ... are needed to preserve life and society (which is also a way of preserving life). Having had my own relatives die in the Holocaust during WWII (Jews in Poland, my grandparents' family although I did not know them), I believe that some wars may be necessary to fight true evil. On the other hand, they must be avoided at every cost, and some wars ... over nationalism, religion, land which all could share ... are not excusable.
I believe that you were there as a peace-keeper, a Bodhisattva's mission. You were protecting innocents. In the heat of the moment, an innocent was killed. It is tragic, it is something to carry always. It also may have been unavoidable in the circumstances, an accident on a terrible peace-keeping mission. It is my belief that one is truly not liable ... in society's law or Buddha Law ... for unintended actions short of true recklessness. Nonetheless, accident or not, we feel the weight of what was done.