The World Wars

TL;DR. In the first half of the twentieth century, two enormous wars tore through the world. World War I (1914 to 1918) grew out of a tangle of rival alliances and a single assassination, then bogged down into years of trench warfare that killed millions. The peace that followed was harsh, and the hardship of the years after it helped dictators rise. World War II (1939 to 1945) was even larger and even deadlier, and it included the Holocaust, the systematic murder of about six million Jews by Nazi Germany. When it ended, the world tried to build new ways to keep such horrors from happening again.

Key takeaways

• World War I pitted the Allies (chiefly France, Russia, and Britain, joined later by Italy and the United States) against the Central Powers (Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria).
• The war collapsed four old empires (German, Austro-Hungarian, Russian, and Ottoman) and redrew the map of Europe.
• The harsh peace, followed by the Great Depression, helped fascism and Nazism take power in Italy and Germany.
• World War II pitted the Axis (Germany, Italy, and Japan) against the Allies (Britain and its empire, the Soviet Union, the United States, France, China, and others).
• The Holocaust was the deliberate, organized Nazi murder of about six million Jews, along with Roma, disabled people, and many others.
• Together the two wars killed tens of millions of people and led to the Nuremberg trials, the United Nations, and the first steps toward uniting Europe.

Main events at a glance

A world primed for war

By the early 1900s, the major countries of Europe had divided themselves into two groups of friends and protectors. These groups were called alliances. An alliance is a promise: if one member is attacked, the others agree to help. The idea was to keep the peace by making any attack too costly to risk. In practice it did the opposite. It meant that a small quarrel between two countries could pull in all of their allies at once.

On one side stood France, Russia, and Britain, an arrangement often called the Triple Entente. The word entente is simply French for "understanding," meaning a friendly agreement to cooperate. On the other side stood Germany and Austria-Hungary, the core of what would become known as the Central Powers. Each side built up large armies and navies and watched the other with suspicion.

Don't be confused: the alliances changed between the two wars. In World War I the Allies (the Entente side) fought the Central Powers, and Italy ended up on the Allied side. In World War II the Allies fought the Axis, and Italy was on the Axis side, against its former partners. Germany was an enemy of the Allies in both wars. Russia is the trickiest case: it fought on the Allied side in World War I, then left, and in World War II the Soviet Union (the country that grew out of Russia) again fought on the Allied side against Germany, even though it would become the West's great rival soon afterward.

World War I: the spark and the slaughter

The trigger came in June 1914 in Sarajevo, a city in the Balkans, a region in southeastern Europe. There a young assassin shot and killed Archduke Franz Ferdinand, the heir to the throne of Austria-Hungary, along with his wife. Austria-Hungary blamed Serbia, a small neighboring country, and prepared to punish it.

What might have stayed a local crisis instead set off the alliances like falling dominoes. Austria-Hungary threatened Serbia. Russia, which saw itself as Serbia's protector, began to mobilize its army. Germany backed Austria-Hungary and declared war on Russia and then on France. When German troops marched through neutral Belgium to reach France, Britain declared war on Germany. Within weeks much of Europe was fighting.

By this point the two camps were clear. The Allies, also called the Entente powers, included France, Russia, and Britain, later joined by Italy and, near the end, the United States. The Central Powers were Germany, Austria-Hungary, the Ottoman Empire (the large state centered on present-day Turkey), and Bulgaria.

Many people on both sides expected a short, glorious war. They were wrong. On the Western Front, the long battle line running through France and Belgium, the armies dug in. Soldiers lived in trenches, which were long ditches cut into the ground, separated from the enemy by a deadly strip of open land covered in barbed wire. To attack meant climbing out and running across that ground into machine-gun fire. Battles could last for months and cost hundreds of thousands of lives while moving the line only a few miles. New weapons made it worse. Machine guns, heavy artillery, poison gas, and eventually tanks and aircraft turned the war into industrial-scale killing.

The fighting was not only in Europe. It spread across the Middle East, Africa, and the seas, and it drew in soldiers and resources from the colonies of the great empires. This is part of why it is called a world war.

Russia leaves, America arrives, Germany falls

By 1917 the war had exhausted everyone, but two events changed its course. In Russia, years of suffering and food shortages led to revolution. The old monarchy fell, and a new government led by revolutionaries called the Bolsheviks took power and pulled Russia out of the war. (This Russian Revolution of 1917 is covered more fully elsewhere; here it matters because it freed Germany to focus on the Western Front.)

In the same year, the United States entered the war on the Allied side, angered by German submarine attacks on ships. American troops, money, and supplies arrived slowly at first and then in growing numbers, tipping the balance against the worn-down Central Powers.

Through 1918 Germany made one last great push and failed. Its allies dropped out one by one, its army fell back, and unrest broke out at home. In November 1918 Germany agreed to an armistice, which is a halt to the fighting. The guns fell silent. The cost was staggering: roughly nine million soldiers had died, along with millions of civilians.

The peace that did not last

In 1919 the victors gathered near Paris and produced the Treaty of Versailles, the main peace agreement with Germany. It was harsh by design. Germany was forced to accept the blame for the war, give up territory and its overseas colonies, sharply limit its armed forces, and pay enormous sums of money to the winners. These payments were called reparations, meant to repair the damage the war had caused. Many Germans felt humiliated and treated unfairly, and that bitterness would matter greatly later.

The war also destroyed empires. Four of them came apart: the German Empire, the Austro-Hungarian Empire, the Russian Empire, and the Ottoman Empire. Out of their ruins came a cluster of new or reshaped countries across central and eastern Europe and the Middle East. The map of Europe in 1920 looked very different from the map of 1914.

To help keep the peace, the victors created the League of Nations, an organization where countries could settle disputes by talking rather than fighting. It was a hopeful idea, but it was weak. It had no army of its own, the United States never joined, and it proved unable to stop determined aggressors in the years to come.

The interwar years: hard times and hard men

The two decades between the wars are called the interwar period. They began with hope but slid toward disaster. In 1929 a financial crash in the United States triggered the Great Depression, a worldwide economic collapse. Banks failed, factories closed, and millions of people lost their jobs and savings. The misery was deep and global.

Hard times made people desperate, and desperate people sometimes turn to leaders who promise order, pride, and revenge. This was the soil in which fascism grew. Fascism is a political movement built on extreme nationalism, obedience to a single strong leader, glorification of the state, and hostility to democracy. In Italy, Benito Mussolini had already taken power in the 1920s and turned the country into a one-party fascist state.

In Germany, the movement took an even more dangerous form. Adolf Hitler led the Nazi Party, whose ideas combined fascism with violent racism, above all a hatred of Jewish people. In 1933 Hitler came to power, and within a short time he destroyed German democracy, silenced opponents, and began rebuilding the military in defiance of the Versailles treaty. The democracies of Britain and France, weary of war and slow to respond, mostly tried to avoid conflict and hoped Hitler could be satisfied. He could not.

A grim preview came in the Spanish Civil War of the late 1930s, in which a fascist-backed side led by Francisco Franco fought against the elected government. Germany and Italy sent help to Franco's side and used the war to test their weapons and tactics. Many observers saw it as a rehearsal for a larger war to come.

World War II: the world at war again

The second great war began in Europe in September 1939, when Germany invaded Poland. Britain and France, who had promised to defend Poland, declared war on Germany. The lines of the new conflict took shape quickly.

On one side were the Axis powers: Germany, Italy, and Japan, later joined by several smaller states. On the other side were the Allies. The Allies eventually included Britain together with its empire and Commonwealth (such as Canada, Australia, India, and others), the Soviet Union from 1941, the United States from 1941, France, China, and many more.

The opening years went badly for the Allies. German forces overran much of Europe with startling speed. In 1940 France fell, and for a time Britain stood essentially alone against Germany in western Europe, holding out through heavy bombing of its cities.

Two events in 1941 turned a European war into a truly global one. First, Germany broke its earlier pact with the Soviet Union and launched a massive invasion of it, code-named Operation Barbarossa. This opened a vast Eastern Front where some of the war's largest and most terrible battles were fought. Second, in December 1941 Japan attacked the United States naval base at Pearl Harbor in Hawaii, bringing the United States fully into the war against both Japan and Germany.

Don't be confused: the Soviet Union fought on the same side as Britain and the United States. Even though the Soviet Union was a communist dictatorship and would become the West's rival in the Cold War soon after, during World War II it was an Ally. Its enormous and costly struggle against Germany on the Eastern Front was one of the main reasons the Allies won.

The tide turns

Slowly the balance shifted. On the Eastern Front, the German advance was finally broken at the Battle of Stalingrad in the Soviet Union, which ended in early 1943 with the surrender of a large German army. From then on, Soviet forces pushed westward toward Germany.

In the west, the great turning point came on D-Day, the 6th of June 1944, when Allied forces crossed the English Channel and landed on the beaches of Normandy in northern France. It was the largest seaborne invasion in history. From there the Allies fought their way inland and began to free western Europe from German control.

Caught between the Soviets advancing from the east and the western Allies from the west, Germany was slowly crushed. In May 1945, with its cities in ruins and Hitler dead by his own hand, Germany surrendered. The war in Europe was over.

The war against Japan in the Pacific continued for several more months. It ended in August 1945, after the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki, weapons of unprecedented destructive power, and the Soviet Union also entered the war against Japan. Japan surrendered, and World War II came to a close.

The Holocaust

Among the darkest facts of the war is the Holocaust. As the Nazis conquered Europe, they carried out the deliberate, organized murder of Jewish people on a continental scale. Jews were stripped of their rights, herded into sealed districts called ghettos, and then deported to camps built for mass killing. About six million Jews, roughly two-thirds of the Jewish population of Europe, were murdered.

The Nazis also targeted and killed many others they considered undesirable or inferior, including Roma people, disabled people, political opponents, Soviet prisoners of war, and others. The Holocaust was not an accident of war or simple cruelty in the heat of battle. It was a planned program of extermination, carried out with bureaucracy, railways, and industry. It remains one of the gravest crimes in human history, and it is remembered so that it will not be repeated or denied.

The cost and the reckoning

The human cost of World War II is almost impossible to take in. Counting soldiers and civilians together, tens of millions of people died, with the Soviet Union and China suffering especially enormous losses. Whole cities were flattened, and many millions more were left homeless, hungry, or driven from their homes.

When the fighting stopped, the victors tried to face what had happened and to build something better. At the Nuremberg trials, held in Germany after the war, leading Nazis were put on trial for their crimes, including the planning and carrying out of the Holocaust. These trials helped establish the idea that individuals can be held responsible under law for crimes against humanity, even when acting under a government's orders.

To replace the failed League of Nations, the world's countries created the United Nations in 1945, an organization meant to prevent future wars, settle disputes, and promote cooperation and human rights. And in Europe, leaders who had seen their continent torn apart twice in thirty years began to ask whether old enemies, especially France and Germany, might be bound together so tightly by trade and shared institutions that war between them would become unthinkable. That resolve toward European integration would shape the decades to come.

The two world wars ended one era and opened another. The old European empires were fading, the United States and the Soviet Union had emerged as superpowers, and a tense new rivalry between them was already beginning.

To see how that rivalry unfolded, and how a divided Europe began to rebuild and unite, read on. 👉 The Cold War and the European Union