- Assassination of Archduke Franz Ferdinand: The assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, on June 28, 1914, in Sarajevo, Bosnia and Herzegovina (then part of Austria-Hungary), is often considered the trigger that set off the chain of events leading to World War I.
- Declaration of War: Following the assassination, Austria-Hungary issued an ultimatum to Serbia, which was perceived as supportive of the assassins. After Serbia partially rejected the ultimatum, Austria-Hungary declared war on Serbia on July 28, 1914.
- Mobilization of Forces: The system of alliances among European powers led to the rapid mobilization of armies and navies. Germany backed Austria-Hungary, while Russia supported Serbia. Germany's invasion of neutral Belgium on August 4, 1914, brought Great Britain into the conflict.
- Stalemate on the Western Front: The war primarily took place on two major fronts: the Western Front, where trench warfare characterized by intense fighting and limited territorial gains, and the Eastern Front, where large-scale battles involving the Russian Empire, Germany, and Austria-Hungary occurred.
- Entry of the United States: In 1917, the United States entered World War I on the side of the Allied Powers (which also included France, Great Britain, Italy, and Russia). The American involvement brought significant resources and manpower to the Allied cause.
- Armistice and End of the War: On November 11, 1918, an armistice was signed between the Allies and Germany, marking the end of World War I. The armistice agreement came into effect at 11:00 AM, leading to a cessation of hostilities.
- Peace Treaties: The Paris Peace Conference convened after the war to negotiate peace treaties with the defeated Central Powers (Germany, Austria-Hungary, Bulgaria, and the Ottoman Empire). The resulting treaties, including the Treaty of Versailles with Germany, had significant territorial, military, and economic consequences.
- Formation of the League of Nations: The war's aftermath saw the establishment of the League of Nations in 1920, an international organization aimed at preventing future wars through collective security and peaceful conflict resolution.