The major conflict defined by the phrase "the war to end all wars" refers to World War I. This war, which lasted from 1914 to 1918, involved many of the world's great powers and resulted in unprecedented levels of destruction and loss of life. The phrase came to symbolize the hope that the war would bring about lasting peace and prevent future global conflicts. However, the harsh peace treaties that followed, particularly the Treaty of Versailles, contributed to lingering tensions, ultimately leading to World War II. Thus, the promise of peace was tragically unfulfilled.