1. World War I began due to rising militarism, alliances, imperialism, and nationalism in Europe that created tensions between countries. 2. The assassination of Archduke Franz Ferdinand by Gavrilo Princip triggered World War I when Austria-Hungary issued a harsh ultimatum to Serbia and Germany backed Austria, leading other countries to join the fighting. 3. Technology advanced rapidly during World War I, including the introduction of tanks, airplanes, chemical weapons, and long-range artillery that increased the lethality and scale of combat.