Although a number of things brought about world war i, there was one event that pushed europe over the edge. what was that event?

World War I (First World War or Great War o WWI) started on July 28, 1914 and ended on November 11, 1918, was a war focused in Europe. The war divided two nations: the Allies consisting of Russian Empire, France, Italy, Japan and United Kingdom or British Empire against the Central powers consisting of Germany, Ottoman Empire and Austria-Hungary. The war, introducing new military technology weapons, had killed an estimation of nine million soldiers in annihilation, bloodshed and massacre.