There are many factors that led to the outbreak of WWI and they all have their importance in history of WWI, but there are three factors that I think are quite important. 1. The Ultimatum sent to Serbia from Austria-Hungary. On July 23, 1914, Austria-Hungary sent an Ultimatum to Serbia with 10 really tough demands; they gave Serbia only two days to revise and expected Serbia to refuse the Ultimatum completely. But the Serbs agreed most of the Ultimatum …showed first 75 words of 605 total…

…showed last 75 words of 605 total…attacked Serbia without any excuse, but it just happened to be a Ultimatum that led to the war between Austria-Hungary and Serbia, which was one of the event that caused Russia to strengthen its army. My final conclusion is: Austria-Hungary's Ultimatum to Serbia caused Russia to prepare their army, which was a excuse for Germany to use the Schlieffen plan and led WWI to its final outbreak (Austria-Hungary's Ultimatum; Russia's army prepare;the Schlieffen plan).