Word War I was fought primarily in Europe and the Middle East between a total of 32 countries: the Allies and the Central Powers. It began on July 28th, 1914, and ended on Nov. 11, 1918, with the Central Powers losing the war.
World War I is also referred to as the Great War or the War of Nations and is often abbreviated as WWI. World War I started as a mere local war between Serbia and Austria-Hungary because a Serbian individual assassinated the Archduke Ferdinand of Austria-Hungary. The countries were already upset with one another, but this move escalated the tension into a full-blown war. Austria declared war on Serbia after the assassination with Germany declaring war on Russia and France a few days later.
The United States entered the foray on April 6, 1917, because the Germans were sinking ships around Britain and sunk a passenger ship without warning. The Lusitania, the passenger ship that was sunk, had 128 Americans onboard, and many were killed. The Germans also sank a French ship that killed more Americans so the United States decided to enter the conflict. President Wilson of the United States wanted to help keep peace throughout the global world, so he helped to create the League of Nations.