I happened to notice, thanks to a cursory web scan, that today is the 100th anniversary of the assassination of the Austrian Archduke Ferdinand by the Serbian nationalist Gavrilo Princip, which set off a chain of disastrous events that led to the First World War. To be fair to Princip, he probably didn't have the whole world war thing in mind when he did it, but that's the way history rocks it.
If there is one thing historians of the 20th century agree upon, it is that this war, a product of arrogance, ignorance and imperialism, was an unparalleled disaster that set in motion the other nasty events of the 20th century. It would be hard to imagine the rise of Nazism without the humiliating defeat of Germany and the punitive treaty that followed. I doubt that the Bolsheviks would have been able to achieve a monopoly of power without the near collapse of Russia the war brought about, which would have also meant no Stalinism.
Ironically, many of the people at the time who opposed the war, such as radicals and socialists, were marginalized and persecuted.
There's not much point in speculating on the "what if?" question. I think some kinds of imperialist/colonial wars were inevitable given the state of the world economy at the time. But it would be hard to imagine a worse course of events, the effects of which we are still feeling.
(Did you guys notice the elegant way in which I avoided ending that lest sentence with a preposition?)
A friend of mine in New Hampshire pointed out in this blog post that yesterday was also the 100th anniversary of the conviction of radical labor organizer and songwriter Joe Hill of the Industrial Workers of the World. Hill would have undoubted protested the war as well had he lived, but he was executed in Utah in 1915 on questionable murder charges. Hill wrote some of the best and funniest labor songs ever written, which some disreputable rabble-rousers, myself included, know by heart.
Here's hoping for a better century to come, although I'm not inclined to bet the farm on it.