why did ww1 start

World War I started primarily due to a complex combination of factors culminating in the assassination of Archduke Franz Ferdinand of Austria- Hungary on June 28, 1914, by a Serbian nationalist. This assassination triggered a chain of events: Austria-Hungary declared war on Serbia; alliances and treaties caused other countries to join the conflict quickly. Germany sided with Austria-Hungary and declared war on Russia and France, while Britain entered the war after Germany invaded Belgium. Underlying causes included imperialism, militarism, nationalism, and a tangled web of alliances and rivalries among European powers. The tensions from European expansionism, arms races, and competing empires heightened the likelihood of a large-scale war.