Short answers, pulled from the story.
World War I ran from the 28th of July 1914 to the 11th of November 1918. The fighting ended with the Armistice of the 11th of November 1918, though the Treaty of Versailles with Germany was not signed until the 28th of June 1919.
World War I was triggered by the assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, and his wife Sophie in Sarajevo on the 28th of June 1914. Gavrilo Princip, a Bosnian Serb, fired the fatal shots after the Archduke's car took a wrong turn.
World War I was fought between two coalitions, the Allies, also called the Entente, and the Central Powers. Major participants included the United Kingdom, France, Russia, Italy, and later the United States on the Allied side, against Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria.
World War I caused an estimated 15 to 22 million military and civilian deaths, ranking it among the deadliest conflicts in human history. The total includes between 9 and 11 million military personnel and an estimated 6 to 13 million civilians, with about 23 million more wounded.
The United States entered World War I on the 6th of April 1917 after Germany resumed unrestricted submarine warfare against Atlantic shipping. The German submarine offensive cost American lives and paralysed trade, creating the support President Woodrow Wilson needed for a declaration of war.
The Treaty of Versailles, signed on the 28th of June 1919, imposed terms on a defeated Germany, which lost significant territory, was disarmed, and was required to pay large reparations. Its Article 231, known as the War Guilt Clause, assigned responsibility for the war and left most Germans humiliated and resentful.
The Treaty of Versailles and German resentment over the War Guilt Clause, reparations, and occupation persisted through the 1920s and 1930s. The League of Nations, established to maintain peace, failed to manage interwar instability, which contributed to the outbreak of World War II in 1939.