A few years ago Steven Pinker wrote the book The Better Angels of Our Nature, in which he argues that violence is declining, at least among industrial nations, and that we are in general becoming nicer. At over 700 pages, I confess that it was too much for me, but I read portions from a book in the library. The book is full of statistics but not that difficult to read and is fairly well written. Here is a TED talk Pinker gave on the book.
There has not been a war between major powers since WW II, which is an historically unprecedented length of time. In the last 400 years, nations have become more democratic, slavery has been mostly eliminated, women have been given the right to vote and civil rights have increased.
The industrial nations have also become more secular. I see two reasons for this. One is that science has largely nudged out religion as a way of explaining the world. The other is that the moral view presented by the Abrahamic religions is hopelessly outdated. They accept slavery, treat women as second class citizens, advocate corporal punishment of children and think that homosexuality is an abomination.
It is fashionable to poke fun of the ideals of the Enlightenment, but I think that in addition to the associated technological changes, there has been a major change in the way we think. Rationality has not yet completely replaced bigotry and superstition, but I think that is the direction we are heading, and in such an environment war no longer makes sense. That is the basis of my optimistic scenario which I previously mentioned.