is there ever a valid reason to start a war? Perhaps there is, but I can't think of one - it worries me.

By my own principles I will say no, there's no ethically justifiable reason to initiate a war. The reason being that if you do justify it, the rationale behind is to reach a (hypothetical) greater good with a sacrifice of lives. This is a dangerous way of thinking because it trivializes suffering and makes it only a means to an end. That is what happened in Soviet Union: tens of millions of their own people killed under the totalitarian system, only in the name of the communist utopia that never came.