World War One changed the entire world and basically set the course of the century.

The Balkans further slid into a divided and contentious region with the breakup of the Austro Hungarian Empire, while Germany lost all of its colonies to Japan and Britain. The British Empire, while larger than ever, was beginning to rot as nationalist movements demanded greater self governance in return for their service to the Empire during the war as well as a broken economy (as Niall Ferguson discusses in "Empire," which while it is biased towards imperialism it is a good history of the British Empire).

Then you have the German economy which was falling apart for some time after the war, and even when it prospered in the late 20s Reparation payments were still a burden to the economy, leading to the rise of Hitler as the Depression brought hard times again.

America enters the world stage as a major military and economic power for the first time. All these effects helped to spark World War Two, which then accelerated the independence movements within European Empires and set America as the foremost world power.

And the effect on the Middle East cannot be underestimated. The collapse of the Ottoman Empire left most of the Middle East as European Mandates, setting the stage for the sectarian violence that would plague the region to this day. The stage was also set for the creation of Israel, the effects of which are self explanatory.

In short WWI set the stage for the world as we know it today.

Sources: Empire by Niall Ferguson The First World War (a documentary in10 parts) "The Great War" by Peter Hart