Unleashed by an archduke's assassination, World War I gradually entangled more and more countries, killing millions of soldiers and civilians and touching multiple continents. In this May 1, 1915 file photo, the British cargo and passenger ship Lusitania as it sets out for England on its last voyage from New York City. The British ocean liner was sunk off Ireland on May 7, 1915 by a German U-Boat, killing 1,150 people, 114 of them Americans. (Source: AP)
The centennial of the end of World War I is reminding Americans of a conflict that is rarely mentioned these days. A British recruiting poster displays Field Marshal Herbert Kitchener in 1914. The poster reads "Your Country Needs You". Kitchener's Army was a group of all-volunteer soldiers formed in the United Kingdom. (Source: AP)
Serbian soldiers take position on the battle line. Some of the first battles of World War One were fought between Serbia and Austria-Hungary around the Cer Mountain region. (Source: AP)
In this August 1914 file photo, German troops stand in formation during the occupation of Brussels. (Source: AP)
The soldiers attend the burial of fellow soldiers and two Canadian nurses, killed in German air raid, during World War One. (Source: AP)
U.S. Army General John J. Pershing, center, inspects French troops at Boulogne, France. (Source: AP)
The U Boat 139 which sank the Lusitania in an unknown location. The British liner was sank on May 7, 1915, by a German submarine off the southern coast of Ireland. About 1,150 men, women and children, perished, 114 of which were Americans. Among those Americans who perished were Charles Frohman, Alfred G. Vanderbilt and Elbert Hubbard. (Source: AP)
The United States army troops stand in the trenches in France during World War One. (Source: AP)
In this Nov. 11, 1918 file photo, people in the streets of Paris, France, celebrate the signing of the Armistice that ended the fighting of World War One. (Source: AP)
The american soldiers from New York, who served on the frontline in Cambria, France, rig up a Liberty Bell to celebrate the signing of the Armistice to end World War One. (Source: AP)