Considering we are in the centenary of World War One and everyone is talking about it, this question sprung to mind as I don't have a clear answer in my head. I know vaguely that in the long term, it was a series of diplomatic clashes between the Great Powers and the disruption of the balance of power in Europe, and in the short term causes, the assassination of Franz Ferdinand and the July Crisis, but could I get a well-informed opinion here?
Finally, something I know the answer to!
Long Term:
Nationalism (pride & ethnic groups wanting their own nations) Imperialism (competition over colonies between Great Britain, Germany, and France)
Militarism (influx in building of military because of industrialization and Germany was increasing their navy which pissed of GB because they had to have 10% more navy ships then all the countries)
Alliances (triple alliance between Germany, Austria Hungary, and Italy; triple entente between Russia, GB, France, and Ireland. This created two sides against each other. All of these alliances wouldn't have been formed if William II of Germany didn't fire Bismarck because he knew his shit and had alliances formed that would prevent war and that were not two sided like these ones)
Short Term:
Assassination of Archduke Franz Ferdinand
This one is a little bit more complicated. Originally many historians blamed the Germans for the war, which is partially true. It is better to look at it as Germany escalating the problem and the dominos all falling after that.
So Franz Ferdinand was the Archduke of Austria Hungary. He was assassinated in Sarajevo, Bosnia by a Serbian terrorist organization which went by the name "The Black Hand."
A-H (Austria Hungary) blamed Serbia and sent them an ultimatum. Serbia contacted Russia who agreed to help them out if the A-H's attacked. At that point A-H realized they needed more power so they contacted Germany who send them what is called a Blank Check, or a promise of unlimited aid.
A-H declared war on Serbia & all hell breaks lose and then Germany goes through with the Schlieffen Plan, which was a monumental disaster and the war just sorta happens.
(Schlieffen Plan was Germany's idea to invade France. Didn't really look good in theory or in practice cause they had to attack France by going through Belgium which was a neutral country and GB was pissed at the Germans....see, it's just the Germans pissing off everyone but not directly their fault...WWII on the other hand is debatable)
Let me know if you need any more information.
Sources:
Paris 1919 (really good book on the Paris Peace Conference)
The First World War: An Illustrated History by A.J.P. Taylor (required to read it by IB 20th century European History teacher
as well as my AP European History & IB 20th century European classes.
edit: I think it's important to note that the Moroccan Crises & Balkan Crisis caused some ambiguity between many nations (France and Germany especially, I think that's why Germany set up the Schlieffen Plan). Lemme know if you need me to explain why these two separate incidents caused issues