This very post can start a war, it can be reasoned, argued, debated. Although an interesting comic pauses us for a moment and quite possibly makes us think really deep thoughts, there still can be answers here.
I think immediately of earlier wars, WWII for instance. There was fascism and expansionism and were known as the axis powers. And those soldiers were stomping across Europe and the Pacific causing genocide and a multitude of problems and atrocities. This was a very big problem for the world. It needed to be stopped. Thus the axis power soldiers and the allied powers soldiers fought each other, killed each other. One in order to continue on, the other to stop their madness in an atempt to restore peace.
This certainly does not excuse many wars either in the backyard against a neighbor or on large scale against countries.
Military men have always asked during times of war "why are we here"? Sometimes there is an answer and sometimes there isn't, perhaps there is an answer but the truth is never known.
I think if a boy were to ask me that question, it might be difficult to answer, but I would try. It may come down to right and wrong, good vs. evil, men who fight for their lives vs. those who think simple might makes right.
So many thougths that you now invoke NMR, I'll be pondering this question of if a boy was to ask me that for the rest of the day.