There were similar arguments before WW I, but the European nations still went to war. All you need is a situation where the people in charge hate their enemies more than they love their children.
. Underestimation of the cost of an early 20th C. war. Some glorious charges, you win, get home in time for dinner.
. A game theory problem in that once you start mobilizing you can't stop. Not starting the war is more dangerous than simply drawing down.
. The simple fun (for leadership) and profits in a war. Pushing pieces around a map is an enjoyable endeavor.
. Interlocking treaties with single points of failure.
etc.