The First World War engulfed the entire globe from 1914 to 1918, with over 70 million men mobilized across multiple empires and countries. It began as a regional conflict due to nationalist tensions and secret alliances in the Balkans but rapidly expanded to include most major world powers. The war was more destructive than initially expected due to modern industrial weapons and tactics like trench warfare. It resulted in immense casualties with little territorial change and ultimately led to the decline of several long-standing empires.