World War I (also known as the First World War or the Great War) was a global conflict fought from 1914 to 1918. It involved most of the world's great powers, assembled in two opposing alliances: the Allies (centered on the Triple Entente of France, Russia, and Britain) and the Central Powers (Germany, Austria-Hungary, and the Ottoman Empire).
Diplomatic Origins
The war's origins lay in a complex web of alliances, imperial rivalries, militarism, and nationalism. The assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914 in Sarajevo triggered a chain of diplomatic and military escalations that led to general war within weeks.
The Outbreak
Austria-Hungary declared war on Serbia on July 28, 1914. Russia mobilized in support of Serbia, leading Germany to declare war on Russia (August 1) and France (August 3). Germany's invasion of neutral Belgium brought Britain into the war on August 4.
The Western Front
After the initial German advance was halted at the Battle of the Marne in September 1914, the Western Front settled into trench warfare stretching from the English Channel to Switzerland. The Battles of Verdun and the Somme in 1916 resulted in massive casualties with little territorial change. The introduction of tanks, poison gas, and aircraft changed the nature of warfare but could not break the stalemate until 1918.
The Eastern Front
The Eastern Front was more mobile than the west. Initial Russian advances into East Prussia were repulsed at Tannenberg (August 1914). By 1917, Russia was exhausted by the war and revolution, leading to the Treaty of Brest-Litovsk in March 1918.
Other Theaters
The war extended to the Middle East (against the Ottoman Empire), Africa (colonial campaigns), and the high seas (including the Battle of Jutland in 1916 and unrestricted submarine warfare). Italy entered the war against Austria-Hungary in 1915. The failed Gallipoli Campaign (1915-1916) attempted to knock the Ottoman Empire out of the war.
US Entry and War's End
The United States entered the war in April 1917, providing fresh manpower and resources. Germany's Spring Offensive of 1918 initially achieved breakthrough but ultimately failed. The Allied Hundred Days Offensive from August 1918 drove the Central Powers back. An armistice was signed on November 11, 1918.
Aftermath
The war resulted in approximately 37 million casualties (military and civilian). It led to the collapse of four empires (German, Austro-Hungarian, Ottoman, and Russian), the Treaty of Versailles, the creation of the League of Nations, and set the stage for World War II.