The most significant event of the twentieth century? Hyperbolic as it may sound, it could well be argued that the assassination of Archduke Franz Ferdinand in Sarajevo by Gavrilo Princip was just so, given the consequences that followed the shooting. ...
Had there been no assassination, Austria would have had no casus bellum with Serbia.
Had there been no First World War, a whole chain of events could not have arisen. Although how long the Romanov dynasty could have survived in Russia is questionable, without a war it is unlikely that the conditions for a revolution would have existed, and especially not a second one to give the Bolsheviks power (after all, the Germans arranged for Lenin’s return from Switzerland as they envisaged him destabilising Russia’s war effort). Without a war Germany would not have had the punitive terms of the Treaty of Versailles to contend with and a sense of being “stabbed in the back” ...
Note: The newspaper reports are from the day after. News of the assassination is on page 13.