https://upload.wikimedia.org/wikipedia/commons/8/8a/Gavrilo_Princip_captured_in_Sarajevo_1914.jpg
In the years leading up to war Britain, Russia and France formed a group called the Triple Entente, whilst Germany, Austria-Hungary and Italy made up another group called the Triple Alliance.
What impact did the agreements between these countries have on the decisions to go to war and the scale of the war to come? Why did Britain partner with Russia and France but not Germany? This World Book Encyclopedia article is a good place to start finding some answers.
The Great War YouTube series is a brilliant resource on WWI. The video below examines several factors that contributed to the scale of WWI but with a focus on the alliances prior to war.
https://www.youtube.com/watch?v=f11CKYB2FCA