[HIGHLIGHTS] It was ignited by a university student, 70 countries participated in it, and 22 million were killed. A look at World War I

The immediate cause of World War I was the assassination of Austrian Crown Prince Franz Ferdinand and his wife by a Serbian student named Gavrilo Princip on June 28, 1914. But scholars monitor a number of indirect reasons, which paved the way for the outbreak of war, most notably the tension in international relations at the beginning of the twentieth century.

The war lasted more than four years, during which it turned from a European war to a world war, and the European war witnessed two periods, the first known as the movement war and the second as trench warfare.