World War I started off as a European quarrel, caused by rivalry between their nations. But then, it spread, and was even fought in Africa and Asia.
What sparked the war was the assassination of Austrian Archduke Franz Ferdinand by a Serb in 1914. Following this, Austria declared war on Serbia. But the war was not going to be restricted to two nations. This was because of the fact that in the previous few decades, the European nations had formed their alliances.
Soon after the above mentioned conflict, Russia prepared to defend Serbia and the mobilisations started. Then Germany declared war on Russia and then on France. Finally, Britain declared war on Germany and since British Empire was spread all over the world, the world war had to erupt.
World War I was fought mostly on land, except for one big naval battle in 1916, and the use of a few war planes. Once the USA joined the war in 1917, on the Allies side, it was going to get difficult for the Central Powers. World War I ended in 1918, but it left a huge loss which would be counted in the darkest chapters of modern history. But it did not stop there… World War II was to be followed next.