The correct answer to this question is The Assassination of Franz Ferdinand. The war began in 1914 after this assassination. Most thought it would be over in a couple of months, but it lasted for years. It ended in 1918, lasting for four years. WW1 is also known as the First World War or the The Great War for Civilization.
The two sides fighting were the Allies and The Central Powers. Countries that made up the Allies were Russia, France, Belgium, and the British Empire. Down the line, the United States joined them. The Central Powers included Turkey, Bulgaria, Germany, Austria, and Hungary.