World War I, also known as The Great War, lasted from 1914 to 1918 and was one of the deadliest conflicts in human history, involving over 30 countries. The war primarily involved two major alliances: the Allied Powers, including the United Kingdom, France, Russia, and later the United States, and the Central Powers, led by Germany, Austria-Hungary, and the Ottoman Empire. The war was sparked by the assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914, which set off a chain reaction of alliances and treaties coming into play.

World War I was marked by trench warfare, technological innovations, and large-scale battles that resulted in the unimaginable loss of life. New weapons, such as machine guns, poison gas, tanks, and aircraft, made the war more destructive than any previous conflict. Major battles like the Battle of the Somme and Verdun became symbols of the horrific nature of modern warfare, with soldiers facing long periods of inaction followed by violent clashes.

The war had profound social, political, and economic impacts. Empires like the Austro-Hungarian Empire, Ottoman Empire, and Russian Empire collapsed, leading to the rise of new nations. The war also contributed to the Russian Revolution of 1917, the signing of the Treaty of Versailles in 1919, and the eventual rise of Nazi Germany, which set the stage for World War II.

The human cost was staggering, with an estimated 10 million military deaths and 7 million civilian casualties. World War I fundamentally altered the political and cultural landscape of Europe and the world, leading to major changes in national borders, society, and the balance of power. The war's legacy of suffering and its unresolved issues laid the groundwork for future conflicts, including World War II.