The First World War, which lasted from 28 July 1914 to 11 November 1918, was a global conflict that changed the world’s political, social, and economic landscape. Several key moments in the war were significant and had long-lasting impacts. One of the starting points of the First World War was the assassination of Archduke Franz Ferdinand of Austria on 28 June 1914. This act sparked a chain of alliances and conflicts, resulting in massive mobilization in Europe. Countries such as Germany, Austria-Hungary, Russia, and France soon became embroiled in increasing tensions. Once the conflict began, the Battle of the Marne in September 1914 was decisive. The Allied victory at the Marne halted the German advance towards Paris and marked an important turning point in the war. The trend of trench warfare began, creating terrible fighting conditions on the Western Front and causing heavy losses. In April 1917, the United States entered the war, which greatly changed the dynamics. With abundant human and material resources, American involvement gave the Allies additional morale and strength. This moment changed the course of the war, giving new hope to the countries involved in the conflict. On the other hand, Russia’s withdrawal from the war after the Bolshevik Revolution in 1917 also influenced the course of the battle. The Treaty of Brest-Litovsk in March 1918 between Germany and Russia allowed Germany to move its troops to the Western Front. The Battle of the Meuse-Argonne in September to November 1918 was one of the largest offensives undertaken by the American Armed Forces. The Allied victory here hastened the end of the war and became a watershed moment in the history of modern warfare. This attack involved more than a million American troops, underscoring their crucial role in ultimately defeating the Central powers. The Armistice signed on 11 November 1918 marked the end of the war. However, the impact of the First World War was felt for decades to come. The Treaty of Versailles in 1919, which ended the conflict, created discontent in Germany and contributed to the rise of Nazism. This war also led to major political and social changes, including the fall of the Austro-Hungarian, Ottoman, Russian, and German empires. New states emerged in Europe, and the political map of the world changed dramatically. On a global level, the First World War encouraged the development of more advanced military technology, including the use of aircraft, tanks and chemical weapons. The horrific experiences of this conflict also fueled the peace and war avoidance movement in the following decades. Ultimately, the First World War served as a reminder to the world of the consequences of extreme nationalism and international conflict. Key moments in this war shaped the history of the 20th century and led to major future developments and conflicts.