The Great War was a military conflict centered on Europe that began in the summer of 1914. The fighting ended in late 1918. This conflict involved most of the world's great powers, assembled in two opposing alliances: the Allies (centred around the Triple Entente) and the Central Powers.
More than 70 million military personnel, including 60 million Europeans, were mobilized in one of the largest wars in history. More than 15 million people were killed, making it also one of the deadliest conflicts in history.
The war is also known as the War to End All Wars.
The assassination on 28 June 1914 of Archduke Franz Ferdinand of Austria, the heir to the throne of Austria-Hungary, is seen as the immediate trigger of the war, though long-term causes, such as imperialistic foreign policies of the great powers of Europe such as the German Empire, the Austro-Hungarian Empire, the Ottoman Empire, the Russian Empire, the British Empire, France, and Italy played a major role. Ferdinand's assassination at the hands of a Yugoslav nationalist resulted in Habsburg ultimatum against the Kingdom of Serbia.
Several alliances that had been formed over the past decades were invoked, so within weeks the major powers were at war; as all had colonies, the conflict soon spread around the world.
The conflict opened with the German invasion of Belgium, Luxembourg and France; the Austro-Hungarian invasion of Serbia and a Russian attack against Prussia. After the German march on Paris was brought to a halt, the Western Front settled into a static battle of attrition with a trench line that changed little until 1917. In the East, the Russian army successfully fought against the Austro-Hungarian forces but was forced back by the German army. Additional fronts opened after the Ottoman Empire joined the war in 1914, Italy and Bulgaria in 1915 and Romania in 1916. The Russian Empire collapsed in 1917, and Russia left the war after the October Revolution later that year. After a 1918 German offensive along the western front, American forces entered the trenches and the German armies were driven back in a series of successful allied offensives. Germany surrendered on Armistice Day, November 11, 1918.
By the war's end, four major imperial powers—the German, Russian, Austro-Hungarian and Ottoman Empires—had been militarily and politically defeated, and the last two ceased to exist. The revolutionized Soviet Union emerged from the Russian Empire, while the map of central Europe was completely redrawn into numerous smaller states. The League of Nations was formed in the hope of preventing another such conflict.
(Content lifted wholesale from Wikipedia.)
After the War, the world faced a new problem - the outbreak of Spanish Flu which ravaged the world, killing millions. In the United States, already heartsick at the loss of so many young men on the field of battle, this new threat caused the federal government to set up roadblocks and cut off interstate travel. This started the divisions and conflicts which created the North America we see today.