First the government got the masses worked up with a propaganda campaign, and then went to war to annex new lands and seize booty. That's what healthy, vigorous countries did, and we had recently run out of Native Americans to pillage. We killed millions in Cuba and the Phillippines and demolished their industry, economy, and infrastructure - a major reason they are poor countries today.
Our Vice President, Teddy Roosevelt, personally led men into battle, killed people defending their homelands, and made millions off the war. So we elected him President.
The world was a rough place 100 years ago.
[NB Anyway I stole the point from the start of Niall Ferguson's The Ascent of Money]
Nationalism was on the rise, new technology gave access to weapons on an unprecedented scale (and weapons production was already well underway in 1913). I won't say it was inevitable, but it did not take much of a spark to set Europe in flames. People have compared it to a forest with tons of deadwood lying around just waiting for the right combination of draught, wind and a little bit of activation energy.
And I don't see that men in power were any more racist, classist, sexist etc than anyone else of their time.
http://www.newstatesman.com/2013/12/real-eton-rifles
Indeed, could argue (as Orwell did) that the public school system trained men for their place in war with the "old lie" of Dulce et decorum est pro patria mori.
[NB For those not in the UK - "public schools" are the self-appointed elite private schools, most people go to state schools]
Do you believe that humans are capable of achieving our aims? Now: do you think we are capable of rationally aiming ourselves towards good aims?
If you assent to both, then what is stopping us?
Part of me desperately wants to agree with that, but then I remember that the world probably looked a pretty decent place in 1913 and look how downright awful and risky the next ~75 years turned out to be.
There is no fundamental law of civilization that guarantees that things will only get better. Having said that, I think that on balance things will keep getting better - but only because a lot of people fight for those improvements based upon observations of where our societies are going wrong and putting up a fight to bring them back on course.