On this day in history, WWI began. Here's everything you were always supposed to know about the Great War but may have never learned.

1. The first World War was a global war centered in Europe that began on July 28, 1914, and ended on November 11, 1918. The war lasted four years, three months and 14 days.

2. Before WWII, WWI was called the Great War, the World War and the War to End All Wars. During the four years of conflict, 135 countries participated in the conflict. More than 15 million people died.

3. WWI involved some of the most significant powers of the world at that time. Two opposing alliances – the Allies and the Central Powers – were at odds with one another. The assassination of Archduke Franz Ferdinand of Austria and his pregnant wife Sophie triggered the start of the war. Ferdinand was the nephew of Emperor Franz Josef and heir to the throne of Austria and Hungary.

4. A Serbian terrorist group, the Black Hand, planned the assassination. The man who shot Ferdinand and his wife, Gavrilo Princip, was a Bosnian revolutionary.

5. Though the assassination triggered the start of WWI, several causes factored into the conflict.

Alliances between countries to maintain the power balance in Europe were tangled and not at all secure. All across Europe, countries were earnestly building up their military forces, battleships and arms stores to regain lost territories from previous conflicts. By the end of the war, the four major European empires – the Russians, the Ottomans, the Germans and the Austro-Hungarian had all collapsed.

Austria-Hungary took over Bosnia, a former Turkish province, in 1909, which angered Serbia. Two years later, Germans protested against the French possession of Morocco.

SARATOGA SPRINGS, N.Y. - Soldiers of the 369th Infantry Regiment man a trench in France during World War I. The Signal Corps photograph collection includes every major aspect of the U.S. Army involvement in World War I.

6. US forces joined WW1 when 128 Americans were killed by a German submarine while aboard the British passenger ship Lusitania. In total, 195 passengers were killed. This put pressure on the U.S. government to enter the war. President Woodrow Wilson wanted peace, but in 1917, Germany announced that their submarines were prepared to sink any ships that approach Britain. Wilson then declared America would enter the war, with the goal of restoring peace to the region. Officially, the war began for US forces on April 6, 1917.

7. U.S. forces spent less than eight months in combat. During that time, 116,000 US service members were killed in action, and 204,000 were wounded. Overall, 8 million service members died during the duration of the war, and 21 million were injured. A total of 65 million military members were mobilized during the war.

8. By 1918, German citizens were protesting against the war. Thousands of German citizens were starving because of British naval blockages. The economy in Germany was beginning to collapse. Then the German navy experienced a significant mutiny, which all but quashed the national resolve to continue with the conflict. German Emperor Kaiser Wilhelm II abdicated on November 9, 1918, which helps to encourage all sides to lay down arms.

9. The peace armistice of WWI was signed on November 11, 1918, in Compiegne, France. One year later, the Treaty of Versailles officially ended the war. This treaty required that Germany accept full responsibility for causing the war. The country was required to make reparations to some of the Allied countries and surrender much of its territory to surrounding countries. Germany was also required to surrender its African colonies and limit the size of its standing military.

10. The Treaty of Versailles also established the League of Nations to help prevent future wars. By 1923, 53 European nations were active members of the League of Nations. However, the U.S. Senate refused to allow the US to participate in the League of Nations.

11. Germany joined the League of Nations in 1926, but much of the German population was resentful of the Treaty of Versailles. Just five years later, Germany (along with Japan) withdrew from the League. Italy followed three years later. Shortly after, German nationalism gave rise to the Nazi party. Some historians argue that WWI never actually ended, only that the conflict paused briefly and that WWII was, in fact, a continuation of the Great War.