There was no 1 catalyst for WWI as there are a number of short term and long term causes.
Short Term Causes:
Assassination of Archduke Franz-Ferdinand by Serbian assassins. This greatly affected the Austro-Hungarian Empire as it now perceived Serbia as a threat. This event also removed the Duke who had been an international voice for peace and stability.
The blank cheque given to Austria-Hungary by Germany, which meant that Germany would give unconditional support to whatever they did, potentially preventing a possible Russian intervention.
Schlieffen Plan, that outlined Germany’s quick invasion of France through Belgium intending to prevent a war on two fronts with Russia and France. However, the British upheld their treaty with Belgium and entered the war on France and Russia’s side starting WWI.
Long Term Causes
Franco-Prussian War of 1870. Deteriorated relations between Germany and France because of Otto von Bismarck’s policy of isolating France from gaining allies. Also, the annexation of Alsace-Lorraine in which the French were itching to take back.
Europe’s alignment into two camps, in which Britain, Russia and France created the Triple Entente and Germany, Austria-Hungary and Italy created the Triple Alliance.
Bosnian Crisis In 1908, Austria-Hungary annexed Bosnia-Herzegovina, weakening Russia and worsening relations between Russia and Austria-Hungary, also increasing pro-Russian sentiment in Serbia.
The Moroccan Crises from 1905 to 1911 over the ownership of Morocco between Germany and France, drew Britain closer to France as German aggression increased.
The Balkan Wars from 1912 to 1913 increased tension between Russia and Austria-Hungary as Serbian power grew and Balkan states fought against the Ottoman Empire for independence and power over the region.
These are not all the causes of WWI but all greatly influenced the chain of events that culminated into WWI.
