Contrariwise, I read an argument once from an historian in the 1930s that WWI wouldn't have happened w/o instantaneous communications, because everyone would've had time to calm down about the Archduke.

I'm a bit dubious, though, because wars had been breaking out for centuries without the help of telegraph, and also in hindsight it looks like many Europeans in the early 20th century were spoiling for a fight, though I've never been sure why.