The main cause of the Great War in 1914, commonly known as World War I, was a complex interplay of nationalist tensions, militarism, alliances, and imperial ambitions. The assassination of Archduke Franz Ferdinand of Austria-Hungary by a Serbian nationalist acted as a catalyst, triggering existing alliances and leading to a rapid escalation of conflict. Major powers entered the war due to pre-established treaties and mobilizations, resulting in a vast conflict involving many nations. Nationalist fervor and rivalries among European powers, combined with militaristic attitudes and imperial competition, created a volatile environment that ultimately led to the outbreak of war.