On June 28, 1914, a young man named Gavrilo Princip killed the heir of the Austrian throne Archduke Franz Ferdinand and his wife in a political assassination that marks the beginning of the Twentieth Century. Princip, an agent of the secret Bosnian nationalist organization known as the Black Hand, stumbled upon the royal couple after a failed assassination attempt earlier that day. It seems it was by accident or sheer dumb luck that this assassination even occurred, and the worst of luck that it triggered the first world war. 
The Great War seems to have exploded on the European continent out of nowhere, the war itself being a tragic accident resulting from human error, but there were several underlining causes that attributed to the outbreak of global conflict indicating that even if Gavrilo Princip failed in his assassination attempt, war would have likely happened sooner or later.
Europe in the early years of the twentieth century saw increased dissent and fear of revolution spurring from emerging ideologies of socialism, liberalism, and nationalism. Whether this was a legitimate fear or not matters less than the fact that European leaders believed it was. Leaders believed that war could amend division at home, giving the political Right and Left a common enemy, and this was partially a correct assumption, for when war did arrive there was a surge in nationalist fervor, and cooperation between labor unions and the bourgeoisie.
However, this was a short-lived phenomenon as disillusionment with the war took its toll after several years. This is not to suggest that war was the intended solution to internal strife, but rather that European leaders believed that war could be an effective propaganda tool which made the decision to escalate conflict easier.
European Alliance System
The European Alliance system, which earlier had provided a sense of security against war, ultimately escalated what had been a local conflict in the Austrian Empire to a global conflict of epic proportions.
- Germany had promised to help Austria if a direct war with Russia ever occurred, and after its alliance with Italy weakened, Germany depended on solidarity with Austria ever more.
- France needed the help of Russia if it ever went to war with Germany, and so to not help Russia after it went to war with Germany would risk isolation in any future conflicts.
- Britain depended on the status quo and was fearful of a rising German naval power, and thus threw its lot in with France.
The result was intercontinental war arising out of the destabilizing region of the Balkans in eastern Europe.
Industrialization of War
Finally, the industrialization of warfare caused war to be waged a great speed. The use of railroads allowed for fast mobilization as “Millions of reservists and mountains of artillery, ammunition, and supplies had to be moved in a few hours by a rail network that first had to be diverted from normal commercial uses.”
The very act of a foreign power mobilizing for war demanded mobilization in return, or else run the risk of a quick defeat. The Great Powers mobilized faster than diplomats could come to a compromise that would negate conflict. The very effectiveness of modern war increased its likelihood, and before Europe could take a step back it descended into the trenches or World War I.
 Paxton, Robert. Europe in the Twentieth Century, 5th ed. (Cengage Learning, Etext Edition, 2011), 42.
 Paxton, Europe in 20th Century, 58.
 Paxton, Europe in 20th Century, 59.
 Paxton, Europe in 20th Century, 59-60.