The Serbian nationalist Gavrilo Princip assassinated Archduke Franz Ferdinand of Austria-Hungary in Sarajevo. The assassination caused Austria-Hungary to declare war on Serbia. Russia had pledged to defend Serbia, while Germany had promised to support Austria-Hungary. France was allied with Russia, so almost all of Europe was quickly engulfed in war.

The Schlieffen Plan was developed between 1905 and 1906. The plan called for a quick victory against France so that German forces could redeploy to the Eastern Front before Russian troops could mobilize. Ultimately, fighting on the Western Front became a stalemate that led to trench warfare.

Britain had guaranteed Belgian neutrality since 1839. German troops crossed into Belgium on August 4, 1914, which led to Britain entering the war.

Known as the “Miracle on the Marne,” the British and French launched a counterattack against German forces. This halted the “Great Retreat” of Allied forces which had begun in August 1914. The Battle of the Marne meant it was unlikely Germany would be able to achieve a quick victory on the Western Front.

Tsingtao had been a German colony since 1898, which the British and Japanese viewed as a threat. The Japanese launched a siege and blockade in August 1914. The British sent several ships to assist. The German garrison held out for two months but Japanese troops overran the defenses in November 1914.

Although prohibited under international law, the German Army deployed 168 tons of chlorine gas against French and colonial troops near the Belgian city of Ypres in May 1915. Because they relied on the prevailing winds to deploy the gas, many German soldiers were also killed or injured during the attack. Chlorine gas destroys the tissue of the eyes and lungs.

The

After the sinking of the

The Isonzo River flows through western Slovenia and northern Italy. The Austrians heavily fortified the region and repulsed a dozen Italian attacks between 1915 and 1917. More than 500,000 men on both sides were killed.

The Ottoman Empire joined the Central Powers and attacked Russia in 1914. British, British Commonwealth, and French troops landed at Gallipoli in April 1915 but the attack quickly lost momentum and bogged down in trench warfare. The Allies withdrew their troops in January 1916.

The Battle of the Somme began in July 1916 and did not end until November. It was one of the bloodiest battles in human history, with more than one million casualties.

The Battle of Jutland occurred in 1916 and was an inconclusive engagement. The German fleet withdrew even though they inflicted more casualties on the British, which lost three battle cruisers while the Germans lost one battlecruiser and one dreadnought.

The Russians launched the Brusilov Offensive in June 1916. The Russians used new tactics to break through the German and Austrian lines. Both the Austrians and Russians suffered significant casualties.

In addition to being one of the bloodiest battles of the war, the Somme also marked a new era of armored warfare. British Mark I tanks were equipped with machine guns and 57mm cannons. Of the 32 tanks that participated in the battle, only nine reached German lines. However, they did eliminate several German strongpoints.

Almost 2.5 million men fought at Verdun. The battle began in February 1916 and ended nine months later. More than 700,000 men were killed or wounded during the battle.

The Germans built the Hindenburg Line during the winter of 1916–1917. It covered 90 miles of territory near Arras in northern France. It included many strong defensive positions and the Allies did not break through until 1918.

Lawrence studied history in college and worked as an archaeologist. He became an intelligence officer in 1914 and served as a liaison between the British Army and Arab rebels. He helped the rebels attack Ottoman railways and capture several cities.

The 369th Infantry Regiment was primarily composed of African-Americans and Puerto Ricans. It was the nation’s first all-Black regiment. They won a number of medals while fighting during the Meuse-Argonne Offensive in 1918.

In 1917, British forces dug tunnels under the German lines at Messines, in Flanders. They placed more than 450 tons of explosives beneath the German defenses and detonated them on June 7, 1917. Approximately 10,000 German soldiers died in the massive explosion. The Allies were able to push back the Germans and recapture territory.

The German foreign secretary Arthur Zimmerman sent a telegram to the German ambassador in Mexico. It offered Mexico financial assistance if its army would attack the United States. The Mexicans ignored the offer, but the British intercepted the message and turned it over to the US government. Zimmerman admitted the message was authentic and America declared war soon afterward.

The United States did not declare war on Germany until April 6, 1917. The declaration came after numerous incidents, including the Zimmerman Telegram and the sinking of the

Russia had suffered a number of defeats during World War I. Political, social, and economic pressure led to a revolution in March 1917. Because a provisional government continued to prosecute the war against Germany, the Bolsheviks took power in November.

Rickenbacker was a race car driver and joined the US army in 1917. He scored 26 kills and later became an airline executive. Bong, Zemke, and Olds were all aces from World War II.

Alvin York was a conscientious objector from Tennessee. After being drafted into the army he was conflicted, but finally decided to serve. He led an attack on a German machine gun nest and eventually captured 132 enemy soldiers and 35 machine guns.

The US Marines suffered heavy losses while attacking German positions in the Belleau Wood in 1918. They captured 1,600 Germans and this battle supposedly marked the origin of the “devil dog” moniker. Several songs and memorials have commemorated the sacrifices of the US Marines at Belleau Wood.

Petain joined the French army in 1876 and was ready to retire when World War I began. He was quickly promoted to general and commanded French forces during the Battle of Verdun. He rotated soldiers out of the front line every two weeks and relied on artillery to pummel German forces.

Haig joined the British Army in 1885 and fought in the Sudan and South Africa. He commanded British troops during the Battle of the Somme and critics labeled him a butcher because of the excessive casualties incurred during the battle. On the first day of the offensive, the British suffered 57,000 casualties, including 19,000 killed.

Pershing joined the US Army in 1886. He became a general and led US forces in a punitive raid against Mexican revolutionary Pancho Villa. He also commanded American troops in Europe during World War I. He visited the tomb of Marquis de Lafayette and declared “Lafayette, we are here.” He retired in 1924 and died in 1948.

Hindenburg joined the army in 1866 and retired as a general in 1911. He returned to active duty in 1914 and commanded German troops on the Eastern Front. He believed defeating Russia should be the first priority—not the Western Front. He later became president of the Weimar Republic and died in 1933.

Richthofen was also known as the Red Baron. He joined the German cavalry in 1911 and joined the flying corps in 1915. He scored 80 victories and became commander of Jagdgeschwader 1—known as the “Flying Circus.” Richthofen was shot down and killed on April 21, 1918.

Civilians created victory gardens to boost morale and bolster the food supply (even through rationing was not required). Many men went overseas to fight, so women began to fill their positions, including jobs in heavy industry. Thousands of African-Americans from the South also migrated to the North in search of jobs. These last two events had a significant impact on postwar American society.

Wilson advocated for the creation of a League of Nations,but the US Senate voted not to join. The United Nations was created after World War II.