The major event that started on July 28, 1914, leading to the involvement of multiple countries in World War I, was the assassination of Archduke Franz Ferdinand of Austria-Hungary. His assassination in Sarajevo by a Bosnian Serb nationalist triggered a series of alliances and mobilizations among European powers. This incident sparked the war, as countries like Germany, Austria-Hungary, Russia, France, and Britain declared war on one another, ultimately engulfing much of the world in a devastating global conflict characterized by trench warfare and unprecedented destruction.