Soldiers of an Australian 4th Division field artillery brigade walk on a duckboard track laid across a muddy, shattered battlefield in Chateau Wood, near Hooge, Belgium, on October 29, 1917. This was during the Battle of Passchendaele, fought by British forces and their allies against Germany for control of territory near Ypres, Belgium. #

Nine European Sovereigns at Windsor for the funeral of King Edward VII in May of 1910, four years before the war began. Standing, from left to right: King Haakon VII of Norway, Tsar Ferdinand of Bulgaria, King Manuel II of Portugal, Kaiser Wilhelm II of the German Empire, King George I of Greece and King Albert I of Belgium. Seated, from left to right: King Alfonso XIII of Spain, King-Emperor George V of the United Kingdom and King Frederick VIII of Denmark. Within the next decade, Kaiser Wilhelm II and Tsar Ferdinand's empires would engage in bloody warfare with the nations led by King Albert I and King George V. The war was also a family affair, as Kaiser Wilhelm II was a first cousin to King George V, and an uncle to King Albert I. Of the remaining monarchs pictured, over the next decade one would be assassinated (Greece), three would keep their nations neutral (Norway, Spain, and Denmark), and two would be forced out of power by revolutions. #

In 1914, Austria-Hungary was a powerful and huge country, larger than Germany, with nearly as many citizens. It had been ruled by Emperor Franz Joseph I since 1848, who had been grooming his nephew, Archduke Franz Ferdinand as the heir to the throne. In this photo, taken in Sarajevo on June 28, 1914, a visiting Archduke Franz Ferdinand and his wife, Czech Countess Sophie Chotek, are departing a reception at City Hall. Earlier that morning, on the way to the hall, their motorcade had been attacked by one of a group of Serbian nationalist assassins, whose bomb damaged one car and injured dozens of bystanders. After this photo was taken, the Archduke and his wife climbed into the open car, headed for a nearby hospital to visit the wounded. Just blocks away though, the car paused to turn around, directly in front of another assassin, who walked up to the car and fired two shots, killing both Franz Ferdinand and his wife. #

Assassin Gavrilo Princip (left) and his victim Archduke Franz Ferdinand, both photographed in 1914. Princip, a 19 year old a Bosnian Serb who killed the Archduke, was recruited along with five others by Danilo Ilic, a friend and fellow Bosnian Serb, who was a member of the Black Hand secret society. Their ultimate goal was the creation of a Serbian nation. The conspiracy, assisted by members of Serbia's military, was quickly uncovered, and the attack became a catalyst that would soon set massive armies marching against each other around the world. All of the assassins were captured and tried. Thirteen received medium-to-short prison sentences, including Princip (who was too young for the death penalty, and received the maximum, a 20 year sentence). Three of the conspirators were executed by hanging. Four years after the assassination, Gavrilo Princip died in prison, brought down by tuberculosis, which was worsened by harsh conditions brought on by the war he helped set in motion. #

A Bosnian Serb nationalist (possibly Gavrilo Princip, more likely bystander Ferdinand Behr), is captured by police and taken to the police station in Sarajevo, on June 28, 1914, following the assassination of Archduke Franz Ferdinand, heir to the Austrian-Hungarian throne, and his wife. #

Shortly after the assassination, Austria-Hungary issued a list of demands to Serbia, demanding they halt all anti-Austro-Hungarian activity, dissolve certain political groups, remove certain political officers, and arrest those within its borders who participated in the assassination, among other things -- with 48 hours to comply. Serbia, with the backing of their ally Russia, politely refused to fully comply, and mobilized their army. Soon after, Austria-Hungary, backed by their ally Germany, declared war on Serbia on July 28 1914. A network of treaties and alliances then kicked in, and within a month's time, Germany, Austria-Hungary, Russia, France, Britain, and Japan had all mobilized their armies and declared war. In this photo, taken in August of 1914, Prussian guard infantry in new field gray uniforms leave Berlin, Germany, heading for the front lines. Girls and women along the way greet and hand flowers to them. #

Belgian soldiers with their bicycles in Boulogne, France, 1914. Belgium asserted neutrality from the start of the conflict, but provided a route into France that the German army coveted, so Germany declared it would "treat her as an enemy", if Belgium did not allow German troops free passage. #

The conflict, called the Great War by those involved, was the first large-scale example of modern warfare - technologies still use in battle today were introduced in large scale forms then, some (like chemical attacks) were outlawed and later viewed as war crimes. The newly-invented aeroplane took its place as an observation platform, a bomber, and an anti-personnel weapon, even as an anti-aircraft defense, shooting down enemy aircraft. Here, French soldiers gather around a priest as he blesses an aircraft on the Western Front, in 1915. #

Between 1914 and the war's end in 1918, more than 65 million soldiers were mobilized worldwide - requiring mountains of supplies and gear. Here, on a table set up outside a steel helmet factory in Lubeck, Germany, a display is set up, showing the varying stages of the helmet-making process for Stahlhelms for the Imperial German Army. #

A Belgian soldier smokes a cigarette during a fight between Dendermonde and Oudegem, Belgium, in 1914. Germany had hoped for a swift victory against France, and invaded Belgium in August of 1914, heading into France. The German army swept through Belgium, but was met with stiffer resistance than it anticipated in France. The Germans approached to within 70 kilometers of Paris, but were pushed back a ways, to a more stable position, which would become battlefields lined with trenches, fought over for years. In this opening month of World War I, hundreds of thousands of soldiers and civilians were killed or wounded -- France suffered its greatest single-day loss on August 22nd, when more than 27,000 soldiers were killed by rifle and machine-gun, thousands more wounded. #

Austro-Hungarian troops executing Serbian civilians, likely ca. 1915. Serbians suffered greatly during the war years, counting more than a million casualties by 1918, including losses in battle, mass executions, and the worst typhus epidemic in history. #

The Japanese fleet off the coast of China in 1914. Japan sided with the United Kingdom and its allies, attacking German interests in the Pacific, including island colonies and leased territories on the Chinese mainland. #

The Salonica (Macedonian) front, Indian troops at a Gas mask drill. Allied forces joined with Serbs to battle armies of the Central Powers and force a stable front throughout most of the war. #

The French battleship Bouvet, in the Dardanelles. It was assigned to escort troop convoys through the Mediterranean at the start of the war. In early 1915, part of a larger group of combined British and French ships sent to clear Turkish defenses of the Dardanelles, Bouvet was hit by at least eight Turkish shells, then struck a mine, which caused so much damage, the ship sank within a few minutes. While a few men survived the sinking and were rescued, nearly 650 went down with the ship. #

"Pill box demolishers" being unloaded on the Western Front. These enormous shells weighed 1,400 lbs. Their explosions made craters over 15 ft. deep and 15 yards across. #

A motorcycle dispatch rider studying the details on a grave marker, while in the background an observation balloon is preparing to ascend. The writing on the marker says in German: "Hier ruhen tapfere franzosische Krieger", or Here rest brave French warriors. #

American soldiers, members of Maryland's 117th Trench Mortar Battery, operating a trench mortar. This gun and crew kept up a continuous fire throughout the raid of March 4, 1918 in Badonviller, Muerthe et Modselle, France. #

British tanks pass dead Germans who were alive before the cavalry advanced a few minutes before the picture was taken. World War I saw the debut of tank warfare, with varying levels of success, mostly poor. Many of the earlier models broke down frequently, or got bogged down in mud, fell into trenches, or, (slow-moving) were directly targeted by artillery. #

Ottoman Turk Machine Gun Corps at Tel esh Sheria Gaza Line, in 1917, part of the Sinai and Palestine Campaign. British troops were battling the the Ottoman Empire (supported by Germany), for control of the Suez Canal, Sinai Peninsula, and Palestine. #

An aerial view of the Hellish moonscape of the Western Front during World War I. Hill of Combres, St. Mihiel Sector, north of Hattonchatel and Vigneulles. Note the criss-cross patterns of multiple generations of trenches, and the thousands of craters left by mortars, artillery, and the detonation of underground mines. #

German soldiers flee a gas attack in Flanders, Belgium, in September of 1917. Chemical weapons were a part of the arsenal of World War I armies from the beginning, ranging from irritating tear gases to painful mustard gas, to lethal agents like phosgene and chlorine. #

British enter Lille, France, in October of 1918, after four years of German occupation. Beginning in the summer of 1918, Allied forces began a series of successful counteroffensives, breaking through German lines and cutting off supply lines to Austro-Hungarian forces. As Autumn approached, the end of the war seemed inevitable. #

The USS Nebraska, a United States Navy battleship, with dazzle camouflage painted on the hull, in Norfolk, Virginia, on April 20, 1918. Dazzle camouflage, widely used during the war years, was designed to make it difficult for an enemy to estimate the range, heading, or speed of a ship, and make it a harder target. #