- Assassination of Archduke Franz Ferdinand: The heir to the Austro-Hungarian throne, Archduke Franz Ferdinand, was assassinated by a Serbian nationalist in Sarajevo on June 28, 1914. This event sparked a diplomatic crisis between Austria-Hungary and Serbia and led to the outbreak of war.
- Declaration of war: Austria-Hungary declared war on Serbia on July 28, 1914, after an ultimatum it gave to Serbia was not fully accepted. Russia, as Serbia's ally, began mobilizing its army in response, leading Germany (Austria-Hungary's ally) to declare war on Russia on August 1 and on France, Russia's ally, the next day.
- Battle of the Marne: Fought from September 5-12, 1914, the Battle of the Marne was one of the most significant battles of the war. It halted the German advance on Paris and marked the end of the war's mobile phase, leading to a stalemate on the Western Front for much of the rest of the war.
- Battle of Gallipoli: The Gallipoli campaign was an attempt by the Allies to take control of the strategically important Dardanelles Strait and capture the Ottoman capital, Constantinople (now Istanbul). The campaign lasted from April 1915 to January 1916 and resulted in a costly defeat for the Allies.
- Battle of Verdun: Lasting from February to December 1916, the Battle of Verdun was one of the longest and bloodiest battles of the war. It resulted in enormous casualties on both sides, with over 700,000 killed.
- Battle of the Somme: The Battle of the Somme, which took place from July 1 to November 18, 1916, was the largest battle of the war on the Western Front. It involved more than 3 million soldiers and resulted in over 1 million casualties, making it one of the deadliest battles in history.
- Entry of the United States: The United States declared war on Germany in April 1917 after Germany resumed unrestricted submarine warfare and following the Zimmermann Telegram, a proposal from Germany to Mexico to form an alliance against the US. The entry of the US significantly impacted the war's outcome.
- Russian Revolution: The Russian Revolution, which began in March 1917 with the abdication of Tsar Nicholas II, led to the rise of the Bolshevik Party and the formation of the Soviet Union. Russia withdrew from the war in March 1918 following the Treaty of Brest-Litovsk, signed with Germany.
- Armistice and End of the War: The war officially ended on November 11, 1918, when an armistice was signed between the Allies and Germany. The armistice took effect at 11 am, and the fighting stopped on all fronts. The war resulted in the deaths of millions of people and had a profound impact on the world, shaping the course of the 20th century.