The first cause of WWI (which was proximate), was the assassination of minor noble Franz Ferdinand, who was from Austria-Hungary, by a Serbian Nationalist. This nationalist assassinating Franz was a result of the already pre-existing ideology during this time of nationalism which contributes as the ultimate cause of WW1. As a result of the assassination, we see an entangling of alliances, that starts with Austria-Hungary and Germany, Germany signs a Blank Cheque to ally with Austria- Hungry and agree to punish Serbia. Russia steps in as an ally to protect Serbia from Germany. Which results in Germany's strategy of the Schlieffen Plan to invade France who was allied with Russia to weaken Russia and avoid a Two-Front War (have enemies on both sides). This system of entangling alliances is what pulls in Italy and Great Britain turning into an all-out Great War as these alliances were made up to 100 years prior with the Congress of Vienna for the purpose of defence.