No & probably(ed).
Anyone can have an opinion on anything.
To write about war's experience, one must have experienced it, or perhaps have accounts from its participants to work with, or observations of how war has affected their lives. However, to write about one's own moral assessment of war, I don't think one must necessarily have experienced it.
It seems to me virtually impossible to have an all-encompassing "opinion on war", except on the deepest personal levels; certainly not on a "policy" level, since wars are waged under so many different circumstances.