Tensions had been brewing throughout Europe—especially in the troubled Balkan region of southeast Europe—for years before World War I actually broke out.

A number of alliances involving European powers, the Ottoman Empire, Russia and other parties had existed for years, but political instability in the Balkans (particularly Bosnia, Serbia and Herzegovina) threatened to destroy these agreements.

The spark that ignited World War I was struck in Sarajevo, Bosnia, where Archduke Franz Ferdinand—heir to the Austro-Hungarian Empire—was shot to death along with his wife, Sophie, by the Serbian nationalist Gavrilo Princip on June 28, 1914. Princip and other nationalists were struggling to end Austro-Hungarian rule over Bosnia and Herzegovina.

The assassination of Franz Ferdinand set off a rapidly escalating chain of events: Austria-Hungary, like many countries around the world, blamed the Serbian government for the attack and hoped to use the incident as justification for settling the question of Serbian nationalism once and for all.

Because mighty Russia supported Serbia, Austria-Hungary waited to declare war until its leaders received assurance from German leader Kaiser Wilhelm II that Germany would support their cause. Austro-Hungarian leaders feared that a Russian intervention would involve Russia’s ally, France, and possibly Great Britain as well.

On July 5, Kaiser Wilhelm secretly pledged his support, giving Austria-Hungary a so-called carte blanche, or “blank check” assurance of Germany’s backing in the case of war. The Dual Monarchy of Austria-Hungary then sent an ultimatum to Serbia, with such harsh terms as to make it almost impossible to accept.

Convinced that Austria-Hungary was readying for war, the Serbian government ordered the Serbian army to mobilize and appealed to Russia for assistance. On July 28, Austria-Hungary declared war on Serbia, and the tenuous peace between Europe’s great powers quickly collapsed.

Within a week, Russia, Belgium, France, Great Britain and Serbia had lined up against Austria-Hungary and Germany, and World War I had begun.

According to an aggressive military strategy known as the Schlieffen Plan (named for its mastermind, German Field Marshal Alfred von Schlieffen), Germany began fighting World War I on two fronts, invading France through neutral Belgium in the west and confronting Russia in the east.

On August 4, 1914, German troops crossed the border into Belgium. In the first battle of World War I, the Germans assaulted the heavily fortified city of Liege, using the most powerful weapons in their arsenal—enormous siege cannons—to capture the city by August 15. The Germans left death and destruction in their wake as they advanced through Belgium toward France, shooting civilians and executing a Belgian priest they had accused of inciting civilian resistance.

In the First Battle of the Marne, fought from September 6-9, 1914, French and British forces confronted the invading German army, which had by then penetrated deep into northeastern France, within 30 miles of Paris. The Allied troops checked the German advance and mounted a successful counterattack, driving the Germans back to the north of the Aisne River.

The defeat meant the end of German plans for a quick victory in France. Both sides dug into trenches, and the Western Front was the setting for a hellish war of attrition that would last more than three years.

Particularly long and costly battles in this campaign were fought at Verdun (February-December 1916) and the Battle of the Somme (July-November 1916). German and French troops suffered close to a million casualties in the Battle of Verdun alone.

The bloodshed on the battlefields of the Western Front, and the difficulties its soldiers had for years after the fighting had ended, inspired such works of art as “All Quiet on the Western Front” by Erich Maria Remarque and “In Flanders Fields” by Canadian doctor Lieutenant-Colonel John McCrae. In the latter poem, McCrae writes from the perspective of the fallen soldiers:

Published in 1915, the poem inspired the use of the poppy as a symbol of remembrance.

Visual artists like Otto Dix of Germany and British painters Wyndham Lewis, Paul Nash and David Bomberg used their firsthand experience as soldiers in World War I to create their art, capturing the anguish of trench warfare and exploring the themes of technology, violence and landscapes decimated by war.

On the Eastern Front of World War I, Russian forces invaded the German-held regions of East Prussia and Poland but were stopped short by German and Austrian forces at the Battle of Tannenberg in late August 1914.

Despite that victory, Russia’s assault forced Germany to move two corps from the Western Front to the Eastern, contributing to the German loss in the Battle of the Marne.

Combined with the fierce Allied resistance in France, the ability of Russia’s huge war machine to mobilize relatively quickly in the east ensured a longer, more grueling conflict instead of the quick victory Germany had hoped to win under the Schlieffen Plan.

From 1914 to 1916, Russia’s army mounted several offensives on World War I’s Eastern Front but was unable to break through German lines.

Defeat on the battlefield, combined with economic instability and the scarcity of food and other essentials, led to mounting discontent among the bulk of Russia’s population, especially the poverty-stricken workers and peasants. This increased hostility was directed toward the imperial regime of Czar Nicholas II and his unpopular German-born wife, Alexandra.

Russia’s simmering instability exploded in the Russian Revolution of 1917, spearheaded by Vladimir Lenin and the Bolsheviks, which ended czarist rule and brought a halt to Russian participation in World War I.

Russia reached an armistice with the Central Powers in early December 1917, freeing German troops to face the remaining Allies on the Western Front.

Woodrow Wilson was president during World War I. At the outbreak of fighting in 1914, the U.S. remained on the sidelines, adopting the policy of neutrality favored by Wilson while continuing to engage in commerce and shipping with European countries on both sides of the conflict.

Neutrality, however, it was increasingly difficult to maintain in the face of Germany’s unchecked submarine aggression against neutral ships, including those carrying passengers. In 1915, Germany declared the waters surrounding the British Isles to be a war zone, and German U-boats sunk several commercial and passenger vessels, including some U.S. ships.

Widespread protest over the sinking by U-boat of the British ocean liner Lusitania—traveling from New York to Liverpool, England with hundreds of American passengers onboard—in May 1915 helped turn the tide of American public opinion against Germany. In February 1917, Congress passed a $250 million arms appropriations bill intended to make the United States ready for war.

Germany sunk four more U.S. merchant ships the following month, and on April 2 Woodrow Wilson appeared before Congress and called for a declaration of war against Germany.

With World War I having effectively settled into a stalemate in Europe, the Allies attempted to score a victory against the Ottoman Empire, which entered the conflict on the side of the Central Powers in late 1914.

After a failed attack on the Dardanelles (the strait linking the Sea of Marmara with the Aegean Sea), Allied forces led by Britain launched a large-scale land invasion of the Gallipoli Peninsula in April 1915. The invasion also proved a dismal failure, and in January 1916 Allied forces staged a full retreat from the shores of the peninsula after suffering 250,000 casualties.

British-led forces also combated the Ottoman Turks in Egypt and Mesopotamia, while in northern Italy, Austrian and Italian troops faced off in a series of 12 battles along the Isonzo River, located at the border between the two nations.

The First Battle of the Isonzo took place in the late spring of 1915, soon after Italy’s entrance into the war on the Allied side. In the Twelfth Battle of the Isonzo, also known as the Battle of Caporetto (October 1917), German reinforcements helped Austria-Hungary win a decisive victory.

After Caporetto, Italy’s allies jumped in to offer increased assistance. British and French—and later, American—troops arrived in the region, and the Allies began to take back the Italian Front.

In the years before World War I, the superiority of Britain’s Royal Navy was unchallenged by any other nation’s fleet, but the Imperial German Navy had made substantial strides in closing the gap between the two naval powers. Germany’s strength on the high seas was also aided by its lethal fleet of U-boat submarines.

After the Battle of Dogger Bank in January 1915, in which the British mounted a surprise attack on German ships in the North Sea, the German navy chose not to confront Britain’s mighty Royal Navy in a major battle for more than a year, preferring to rest the bulk of its naval strategy on its U-boats.

The biggest naval engagement of World War I, the Battle of Jutland (May 1916) left British naval superiority on the North Sea intact, and Germany would make no further attempts to break an Allied naval blockade for the remainder of the war.

World War I was the first major conflict to harness the power of planes. Though not as impactful as the British Royal Navy or Germany’s U-boats, the use of planes in World War I presaged their later, pivotal role in military conflicts around the globe.

At the dawn of World War I, aviation was a relatively new field; the Wright brothers took their first sustained flight just eleven years before, in 1903. Aircraft were initially used primarily for reconnaissance missions. During the First Battle of the Marne, information passed from pilots allowed the allies to exploit weak spots in the German lines, helping the Allies to push Germany out of France.

The first machine guns were successfully mounted on planes in June of 1912 in the United States, but were imperfect; if timed incorrectly, a bullet could easily destroy the propeller of the plane it came from. The Morane-Saulnier L, a French plane, provided a solution: The propeller was armored with deflector wedges that prevented bullets from hitting it. The Morane-Saulnier Type L was used by the French, the British Royal Flying Corps (part of the Army), the British Royal Navy Air Service and the Imperial Russian Air Service. The British Bristol Type 22 was another popular model used for both reconnaissance work and as a fighter plane.

Dutch inventor Anthony Fokker improved upon the French deflector system in 1915. His “interrupter” synchronized the firing of the guns with the plane’s propeller to avoid collisions. Though his most popular plane during WWI was the single-seat Fokker Eindecker, Fokker created over 40 kinds of airplanes for the Germans.

The Allies debuted the Handley-Page HP O/400, the first two-engine bomber, in 1915. As aerial technology progressed, long-range heavy bombers like Germany’s Gotha G.V. (first introduced in 1917) were used to strike cities like London. Their speed and maneuverability proved to be far deadlier than Germany’s earlier Zeppelin raids.

By the war’s end, the Allies were producing five times more aircraft than the Germans. On April 1, 1918, the British created the Royal Air Force, or RAF, the first air force to be a separate military branch independent from the navy or army.

With Germany able to build up its strength on the Western Front after the armistice with Russia, Allied troops struggled to hold off another German offensive until promised reinforcements from the United States were able to arrive.

On July 15, 1918, German troops launched what would become the last German offensive of the war, attacking French forces (joined by 85,000 American troops as well as some of the British Expeditionary Force) in the Second Battle of the Marne. The Allies successfully pushed back the German offensive and launched their own counteroffensive just three days later.

After suffering massive casualties, Germany was forced to call off a planned offensive further north, in the Flanders region stretching between France and Belgium, which was envisioned as Germany’s best hope of victory.

The Second Battle of the Marne turned the tide of war decisively towards the Allies, who were able to regain much of France and Belgium in the months that followed.

By the time World War I began, there were four all-Black regiments in the U.S. military: the 24th and 25th Infantry and the 9th and 10th Cavalry. All four regiments comprised of celebrated soldiers who fought in the Spanish-American War and American-Indian Wars, and served in the American territories. But they were not deployed for overseas combat in World War I.

Blacks serving alongside white soldiers on the front lines in Europe was inconceivable to the U.S. military. Instead, the first African American troops sent overseas served in segregated labor battalions, restricted to menial roles in the Army and Navy, and shutout of the Marines, entirely. Their duties mostly included unloading ships, transporting materials from train depots, bases and ports, digging trenches, cooking and maintenance, removing barbed wire and inoperable equipment, and burying soldiers.

Facing criticism from the Black community and civil rights organizations for its quotas and treatment of African American soldiers in the war effort, the military formed two Black combat units in 1917, the 92nd and 93rd Divisions. Trained separately and inadequately in the United States, the divisions fared differently in the war. The 92nd faced criticism for their performance in the Meuse-Argonne campaign in September 1918. The 93rd Division, however, had more success.

With dwindling armies, France asked America for reinforcements, and General John Pershing, commander of the American Expeditionary Forces, sent regiments in the 93 Division to over, since France had experience fighting alongside Black soldiers from their Senegalese French Colonial army. The 93 Division’s 369 regiment, nicknamed the Harlem Hellfighters, fought so gallantly, with a total of 191 days on the front lines, longer than any AEF regiment, that France awarded them the Croix de Guerre for their heroism. More than 350,000 African American soldiers would serve in World War I in various capacities.

By the fall of 1918, the Central Powers were unraveling on all fronts.

Despite the Turkish victory at Gallipoli, later defeats by invading forces and an Arab revolt that destroyed the Ottoman economy and devastated its land, and the Turks signed a treaty with the Allies in late October 1918.

Austria-Hungary, dissolving from within due to growing nationalist movements among its diverse population, reached an armistice on November 4. Facing dwindling resources on the battlefield, discontent on the homefront and the surrender of its allies, Germany was finally forced to seek an armistice on November 11, 1918, ending World War I.

At the Paris Peace Conference in 1919, Allied leaders stated their desire to build a post-war world that would safeguard itself against future conflicts of such a devastating scale.

Some hopeful participants had even begun calling World War I “the War to End All Wars.” But the Treaty of Versailles, signed on June 28, 1919, would not achieve that lofty goal.

Saddled with war guilt, heavy reparations and denied entrance into the League of Nations, Germany felt tricked into signing the treaty, having believed any peace would be a “peace without victory,” as put forward by President Wilson in his famous Fourteen Points speech of January 1918.

As the years passed, hatred of the Versailles treaty and its authors settled into a smoldering resentment in Germany that would, two decades later, be counted among the causes of World War II.

World War I took the lives of more than 9 million soldiers; 21 million more were wounded. Civilian casualties numbered close to 10 million. The two nations most affected were Germany and France, each of which sent some 80 percent of their male populations between the ages of 15 and 49 into battle.

The political disruption surrounding World War I also contributed to the fall of four venerable imperial dynasties: Germany, Austria-Hungary, Russia and Turkey.

The World War I period brought about massive social upheaval, as millions of women entered the workforce to replace men who went to war and those who never came back. The first global war also helped to spread one of the world’s deadliest global pandemics, the Spanish flu epidemic of 1918, which killed an estimated 20 to 50 million people.

World War I has also been referred to as “the first modern war.” Many of the technologies now associated with military conflict—machine guns, tanks, aerial combat and radio communications—were introduced on a massive scale during World War I.

The severe effects that chemical weapons such as mustard gas and phosgene had on soldiers and civilians during World War I galvanized public and military attitudes against their continued use. The Geneva Convention agreements, signed in 1925, restricted the use of chemical and biological agents in warfare and remain in effect today.