Archduke Franz Ferdinand of Austria and his wife, Sophie, leave the Sarajevo Guildhall after reading a speech on June 28, 1914. Five minutes later, they were assassinated. Their assassination precipitated Austria-Hungary's declaration of war against Serbia which later kicked off World War I.
A regiment of famous alpine cyclists occupy a garrison during the Battle of the Somme.
This is one of the last photos of German pilot Richard Scholl before his death, which were collected and sent to his father. Scholl was reported missing in September 1918.
French soldiers on the battlefield during an offensive on the French fortress of Verdun. In total, more than 700,000 people were killed or injured on both the French and German sides during this battle, with casualties split almost evenly between them.
Some of the men of the 369th infantry regimen from New York. The U.S. did not participate in the World War I until late 1917. Before then, an American soldier named Harry Butters had faked his British identity so that he could help their armed forces in the war. Later, British Prime Minister Winston Churchill wrote a memorial to Butters in the newspaper, writing "We realize his nobility in coming to the help of another country entirely of his own free will."
A French soldier at the Battle of Verdun wears a gas mask. Historians have dubbed World War I "modern history's first battle of attrition," in which the goal was simply to take as many enemy lives as possible, no matter the time or the cost. Brutal methods like flamethrowers and poison gas were often used to achieve that goal.
French soldiers launch gas and flame attacks against German troops in Flanders, Belgium.
An Armenian woman kneels beside her dead child in Syria during the Battle of Aleppo in 1918 after Prince Feisal's armed forces captured the city in the last days of the Sinai and Palestine Campaign in the First World War.
The average soldier had to carry 66 pounds of equipment during the Battle of the Somme. But major new technologies for warfare were also introduced in this battle, including the first use of tanks in combat.
British 55th Division troops blinded by tear gas await treatment at an advanced dressing station near Bethune during the Battle of Estaires on April 10, 1918.
Soldiers with anti-aircraft gun during a battle in the First World War.
Gas-masked men of the British Machine Gun Corps with a Vickers machine gun during the first battle of the Somme. It was the first instance of chemical weaponry use.
The French cavalry cross a swollen stream on the front lines of the Battle of the Somme which began on July 1, 1916. The battle remains one of the bloodiest in the history of the British armed forces which lost roughly 60 percent of its troops on the first day alone.
The Battle of Verdun was grisly. According to one French soldier whose unit was bombarded by a German artillery attack, the horrors were unimaginable: "I arrived there with 175 men... I left with 34, several half mad... not replying anymore when I spoke to them."
Doctors would use masks to cover areas near the eye for those severely injured in the war. The eyeglasses this man is wearing aren't meant to improve his vision but to hold the mask in place. The picture on the left shows what the man looks like without his mask post-surgery.
French soldiers take advantage of a peaceful moment on the Western Front to have a meal, complete with flowers and a bottle of wine.
French troops wearing an early form of gas masks in the trenches during the Second Battle of Ypres. This was during Germany's first mass-use of poison gas on the Western Front.
The arrest of Gavrilo Princip, the 19-year-old who assassinated Austrian Archduke Franz Ferdinand. Princip and his accomplices were arrested as members of a Serbian nationalist secret society which led eventually to World War I.
Thousands of German soldiers arriving at a prisoner of war camp.
A unit of German soldiers during World War I with a young Adolf Hitler supposedly on the left.
A woman offers a flower to Indian soldiers fighting on behalf of the British Empire. Giving flowers to soldiers was a traditional sign of welcome and support.
Canadian soldier with burns caused by mustard gas.
Workers amid rows and rows of shells in a large warehouse at the National Filling Factory in the former village of Chilwell. The facility was a UK-owned explosives filling factory.
Soldiers playing football in no man’s land during The Christmas Truce, a series of unofficial ceasefires along the Western Front of World War I around Christmas 1914.
Soldiers of the Royal Irish Rifles, an infantry rifle regiment of the British Army, rest during the beginning hours of the Battle of the Somme. Besides World War I, the regiment also served in the Second World War, the Korean War, and the Second Boer War.
A sentry in the trenches looking through an improvised periscope at the Somme.
A skull dubbed "The Crown Prince" serves as a nighttime point of reference for soldiers fighting in the Battle of Verdun.
Two U.S. soldiers wear gas masks while walking through plumes of smoke. While poison gas was responsible for less than one percent of deaths during the war, the new chemical warfare created a psychological terror that had never been known before.
War cinema crew working on the Western Front.
Wounded soldiers after the recapture of Fort Vaux during the Battle of Verdun. The battle lasted 303 days in 1916.
"This is a war to end all wars," is one of the more famous quotes widely associated with the carnage of World War I. It was made famous by President Woodrow Wilson, but he was not the first one who used it. The British futurist writer and social commentator H.G. Wells invented the phrase in his article 'The War That Will End War' which was published in The Daily News on Aug. 14, 1914, predicting that it would be the last war of its kind. He was, of course, gravely mistaken.
Delegates of Germany and the Allies sign the Treaty of Versailles in the former palace's famous Hall of Mirrors. The treaty brought an end to World War I on June 28, 1919.