Final answer:
The immediate cause of World War I was the assassination of Archduke Franz Ferdinand by a Serbian nationalist, set against a backdrop of the MAIN factors (Militarism, Alliances, Imperialism, and Nationalism), which led to a chain reaction of war declarations among the European powers.
Step-by-step explanation
The immediate cause of World War I was the assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914. The archduke, heir to the Austro-Hungarian throne and proponent of peaceful diplomacy, was killed by Gavrilo Princip, a Serbian nationalist, which led to a chain reaction among the Great Powers of Europe. The tangled web of alliances ensured that the assassination of the archduke caused Austria-Hungary to declare war on Serbia, which then triggered a series of declarations of war among the European powers. This singular event, set against the backdrop of intense Militarism, Alliances, Imperialism, and Nationalism (MAIN) across the continent, sparked what would become known as the 'Great War' or World War I.
While the assassination was the spark, the combustible environment was created by numerous factors. These included the divisions of Africa at the Berlin Conference in 1885, nationalist movements, mutual defense alliances with complex obligations, and Great Power rivalries that had been simmering for years. These long-standing issues made the outbreak of war seem almost inevitable, with the archduke’s assassination merely being the catalyst.