World War I started primarily due to a complex mix of political, territorial, and economic rivalries among the great powers of Europe, militarism, alliances, imperialism, and nationalism, all of which created a tense and volatile environment. The immediate trigger was the assassination of Archduke Franz Ferdinand of Austria-Hungary by a Bosnian Serb nationalist on June 28, 1914. Austria-Hungary blamed Serbia and declared war, leading to a chain reaction of alliance obligations and mobilizations: Russia supported Serbia, Germany supported Austria-Hungary, and soon much of Europe was drawn into the conflict. Germany's invasion of Belgium brought Britain into the war, resulting in a full-scale global conflict. This unraveling was intensified by arms races, imperial competition, and nationalist sentiments, making a large- scale war almost inevitable by 1914.