World War II did not start overnight. A chain of political mistakes, economic collapse, and weak responses to rising threats set the stage for the deadliest conflict in history. These cards break down the key causes so you can study fast and keep the big picture clear. Related: Political Systems in World War II and Military Tactics & Strategy in World War II.
Treaty of Versailles
The peace deal that ended World War I was supposed to prevent another massive war, but the terms it forced on Germany created deep anger that lasted for decades. That resentment gave ambitious leaders a ready audience and a long list of grievances to exploit.
FactsthatCrunchI looked around that hall and saw the German delegates sign their names, and I thought of what this moment would mean for their country. The pen scratched across the paper like a sentence being passed. The terms were harsh, and I could not believe any nation could bear such a weight for long. I feared that day would plant the seeds of another war.
Nicolson recorded his impressions of the signing ceremony at the Palace of Versailles on June 28th, 1919, in his diary and later in his memoir 'Peacemaking 1919'.
FactsthatCrunchGermany was not allowed to negotiate any part of the Treaty of Versailles and was simply handed the finished document to sign.
The Allied powers wrote the treaty's terms among themselves in Paris and gave Germany only three weeks to respond in writing. German leaders called the treaty a 'diktat,' meaning a dictated peace, because they had no seat at the table. This humiliation angered many Germans and made it easier for later leaders to build popular support by promising to tear up the agreement.
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Black & whiteColourFactsthatCrunchThe Treaty of Versailles forced Germany to accept full blame for World War I under Article 231, known as the 'War Guilt Clause.'
Article 231 of the Treaty of Versailles placed sole blame for the war on Germany. This humiliation made many Germans ready to support leaders who promised to restore national pride.
FactsthatCrunchAppeasement is a foreign policy where a country gives concessions to an aggressive power in hopes of avoiding war. Leaders believed that meeting demands would satisfy the aggressor and keep the peace.
"When British Prime Minister Neville Chamberlain agreed at Munich on September 30th, 1938 to let Germany take the Sudetenland region of Czechoslovakia, he called it 'peace for our time,' but the policy of appeasement only encouraged Hitler to demand more territory."
FactsthatCrunchBoth events happened in 1919, and they shaped Germany's future together. The treaty placed heavy burdens on Germany just as the new Weimar Republic was trying to establish itself, making the young government deeply unpopular from the start.
The Weimar Republic was officially proclaimed on November 9th, 1918, and adopted its constitution on August 11th, 1919, the same year the treaty was signed, leaving its leaders to take the blame for accepting the peace terms.
FactsthatCrunchWe will not accept a peace that turns Germany into a slave nation.
Hitler used this kind of language to build public anger over the Treaty of Versailles and to win support for tearing up its terms, which helped push Europe toward a second world war.
FactsthatCrunchThe name given to Article 231 of the Treaty of Versailles, which forced Germany to accept full blame for starting World War I and provided the legal basis for demanding reparations payments.
"The War Guilt Clause made many Germans furious because they felt the country had been unfairly singled out for punishment."
Payments that a defeated country is required to make to cover damages caused during a war, as ordered by the winning powers at the Paris Peace Conference.
"The reparations Germany owed under the Treaty of Versailles placed a heavy financial burden on its struggling economy throughout the 1920s."
The process of removing or sharply limiting military forces from a specific area or country, such as the Rhineland region along Germany's western border, as required by the treaty.
"The demilitarization of the Rhineland was meant to keep Germany from threatening France, but Adolf Hitler ordered troops back into the region on March 7th, 1936."
FactsthatCrunchWe shall have to work hard and unremittingly to tear from the pages of history the name and the shame of this dictated peace.
German Chancellor Philipp Scheidemann, Berlin, May 12th, 1919
Scheidemann spoke these words to the German National Assembly after the Allied powers presented the final treaty terms to Germany. Germany had not been allowed to negotiate the terms, which made many Germans feel humiliated and angry. This reaction became a major grievance that nationalist leaders, including Adolf Hitler, used to build political support throughout the 1920s and 1930s.
FactsthatCrunchThe Treaty of Versailles ended World War I, but its terms shaped the tensions that led to World War II.
FactsthatCrunchKeynes believed the Allied leaders gave "politics precedence over economics" and left no room for Germany's economic recovery. He resigned his Treasury post in protest on May 26th, 1919, before the treaty was signed, then published his warnings in The Economic Consequences of the Peace on December 8th, 1919. He predicted that the harsh terms would ruin Germany financially and push Europe toward another war.
FactsthatCrunchFrench and British leaders saw Article 231 as a fair statement of legal responsibility. It gave the Allies a clear basis to demand payment for the enormous damage Germany had caused. Georges Clemenceau, France's prime minister, pushed hard for the clause because France had suffered the worst fighting on its own soil.
Most Germans felt that forcing Germany to accept sole blame for the war was deeply unjust. Many ordinary citizens and politicians alike saw the clause as a national humiliation, not a reflection of history. This anger made it easier for radical voices to argue that Germany had been stabbed in the back by its own leaders at the peace table.
FactsthatCrunchFactsthatCrunchWhat were the main military restrictions placed on Germany by the Treaty of Versailles?
League of Nations
When the world tried to build an organization that could stop wars before they started, the results were far weaker than anyone had hoped. The gap between what the League promised and what it could actually enforce left aggressive nations room to push boundaries without facing real consequences.
FactsthatCrunchThe League of Nations was an international organization set up in 1920 to keep peace between countries by giving them a shared set of rules and a place to solve disputes.
FactsthatCrunchI watched the Council chamber fall silent when the Ethiopian delegation rose to speak. The great powers sat unmoved as Emperor Haile Selassie warned them that his country was being bombed and gassed by Italian aircraft. I could see that the words reached their ears but not their will, and I left Geneva that day believing the League had chosen comfort over conscience.
Noel-Baker was present at the League of Nations Council sessions in Geneva in 1935 and 1936 during the League's debate over Italy's invasion of Ethiopia, and he later described the atmosphere in those chambers in his writings and speeches.
FactsthatCrunchThe League of Nations holds its first meeting
The League of Nations met for the first time on January 10th, 1920, in Paris, France. It was created to settle disputes between countries peacefully and prevent another world war. The United States never joined, which left the League too weak to stop aggressive nations in the 1930s.
FactsthatCrunchWhen Japan left the League of Nations on March 27th, 1933, it became the first major world power to walk out of the organization.
Japan left after the League condemned its invasion of Manchuria in 1931. The withdrawal showed that the League could not force powerful nations to follow its decisions. This made other aggressive countries less afraid of the League and weakened its ability to keep the peace.
FactsthatCrunchFactsthatCrunchAppeasement is a foreign policy strategy in which a country gives in to the demands of an aggressive power in order to avoid war. Appeasement means making concessions, giving something up, to satisfy a stronger or threatening nation and keep the peace. Leaders who use this strategy hope that meeting demands early will prevent a larger conflict later.
"When British Prime Minister Neville Chamberlain signed the Munich Agreement on September 30th, 1938, letting Germany take the Sudetenland region of Czechoslovakia, he called it 'peace for our time,' but many historians say this act of appeasement only encouraged Adolf Hitler to demand more territory."
FactsthatCrunchThe League of Nations failed to stop aggression in the 1930s largely because key powers were absent or unwilling to act, making collective security impossible in practice.
FactsthatCrunchI believe it is peace for our time.
Chamberlain said this to a crowd outside 10 Downing Street after signing the Munich Agreement, and the phrase later came to stand for the failure of appeasement as a way to stop Hitler.
FactsthatCrunchThe League has failed to stop the war. It has failed to protect the victim of aggression.
Haile Selassie I, Emperor of Ethiopia, address to the League of Nations Assembly, Geneva, June 30th, 1936
Haile Selassie spoke to the League of Nations after Italy invaded Ethiopia in October 1935. The League had imposed some sanctions on Italy, but key items like oil were left off the list, and the measures did not stop the war. His speech made clear to the world that the League could not protect smaller nations from powerful aggressors, which weakened confidence in collective security and encouraged further acts of aggression in the years that followed.
FactsthatCrunchAfter Italy invaded Ethiopia on October 3rd, 1935, Haile Selassie traveled to Geneva and spoke before the League assembly in Amharic, the first head of state to address the body in person. He warned that the League's failure to stop the attack on his country meant "it will be you tomorrow", telling the world's governments that unchecked aggression would spread. The League refused to act, and most member states recognized Italy's conquest, proving his warning right.
Maginot Line
France poured enormous resources into a chain of fortifications meant to make another German invasion impossible, and for a while it seemed like a reasonable bet. The problem was that a fixed wall could only protect the ground directly behind it, and an enemy willing to go around it faced little resistance.
FactsthatCrunchWhy did France build the Maginot Line, and did it stop Germany in World War II?
France built the Maginot Line in the 1930s to protect its border with Germany after the huge losses it suffered in World War I. The line stretched about 280 miles (450 kilometers) and included underground forts, gun positions, and troop quarters. Germany bypassed the line in May 1940 by attacking through Belgium and the Ardennes forest, leaving the fortifications useless.
FactsthatCrunchI walked through the great concrete corridors beneath the ground and felt the walls tremble with the hum of ventilation machines. The casemates smelled of oil and damp stone, and the air never truly felt clean. Soldiers lived and slept far below the surface, sometimes for weeks at a time without seeing daylight. We believed the line made France untouchable, and that belief was complete.
Maginot inspected the early construction works along the Franco-German border in the late 1920s and spoke to journalists and officials about the project before his death on January 7th, 1932.
FactsthatCrunchThe Maginot Line stretched about 280 miles (450 km) along France's border with Germany, roughly the driving distance from New York City to Boston and back.
France built the Maginot Line through the 1930s to guard against a German ground attack across its eastern border. The fortifications ran through roughly 280 miles (450 km) of northeastern France, containing over 500 separate structures, including bunkers, gun batteries, and underground rail tunnels. When Germany invaded in May 1940, its forces went around the line through Belgium and the Ardennes forest, making the massive barrier largely useless.
FactsthatCrunchFortifications are structures built to defend a territory from enemy attack. They include walls, bunkers, trenches, and barriers. Nations build them to protect borders and slow down an invading army. In the years before World War II, several countries invested heavily in fortification lines, believing that strong fixed defenses could prevent war or limit its damage.
"France completed the Maginot Line by 1936, a chain of concrete forts and gun positions stretching roughly 280 miles (450 kilometers) along its border with Germany, built to stop a German invasion."
FactsthatCrunchFrance built the Maginot Line along its border with Germany in the 1930s, hoping it would stop any future invasion.
Great Depression
The global economic collapse that began in 1929 did more than wipe out savings and close factories. It shook people’s faith in democratic governments and made desperate populations across Europe far more willing to listen to leaders who promised simple answers to complicated problems.
FactsthatCrunchDuring the Great Depression, American industrial output fell by about 46 percent between 1929 and 1932, the largest collapse in U.S. manufacturing on record.
The steep drop in production left millions of workers without jobs and caused banks across the country to fail. Germany, which depended heavily on American loans to rebuild its economy after World War I, was hit especially hard when those loans dried up. Mass unemployment and economic despair in Germany helped extremist parties, including the Nazi Party, gain wide public support through the early 1930s.
FactsthatCrunchThe Great Depression spread so fast that U.S. unemployment hit roughly 25 percent by 1933, yet Germany's rate reached nearly 30 percent that same year, making its economic pain even sharper than America's.
When the U.S. stock market crashed in October 1929, American banks that had lent heavily to Germany called in those loans. German factories shut down quickly, and millions of workers lost their jobs within just a few years. That deep economic suffering made many Germans desperate for strong leadership and fast solutions, which helped radical political parties gain votes and power.
FactsthatCrunchThe Great Depression wiped out tax revenue across Europe, forcing governments to cut military and public spending at a time when political extremists were promising voters bold economic solutions.
The Great Depression did not cause World War II on its own, but it made voters across the world ready to accept radical leaders who promised to restore national pride and economic strength. That shift in public mood gave authoritarian governments the popular support they needed to rearm and expand.
FactsthatCrunchA recession is a period when a country's economy shrinks for at least two back-to-back quarters, meaning businesses earn less, jobs disappear, and people spend less money. In the context of World War II, the word often applies to the broader economic collapse of the 1930s, which weakened democratic governments and made voters more willing to accept extreme leaders who promised quick fixes. Germany's economy had already been damaged by war debts and the terms of the Treaty of Versailles, and the global downturn of the Great Depression pushed unemployment even higher. A struggling economy can reduce military budgets, lower morale, and create political openings for leaders who promote aggressive foreign policies as a distraction from hardship at home.
"After the Wall Street Crash of October 1929, the recession spread to Europe so quickly that by 1932 German industrial output had dropped to roughly half of what it had been in 1928, leaving millions without work and making radical political parties far more appealing."
FactsthatCrunchIn early 1933, thousands of American banks had already collapsed, and mass unemployment had spread fear across the democratic world. That same year, economic desperation in Germany helped push voters toward Adolf Hitler, who became Chancellor on January 30th, 1933.
By 1933, roughly 4,000 American banks had failed since 1929, and Germany's economic collapse under the Depression had created the instability that extreme political parties used to gain power.
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FactsthatCrunchWhich American law, passed on June 17th, 1930, raised taxes on imported goods and made the Great Depression worse in Europe?
Adolf Hitler’s Rise to Power
FactsthatCrunchI stood in the crowd at the Sportpalast in Berlin on the 4th of February, 1933, just days after Hitler became Chancellor, and the noise that rose from tens of thousands of throats was unlike anything I had ever heard in a public hall. People around me wept and reached out their hands toward the stage as though they were witnessing something sacred. I had covered political rallies for years, but this was not a rally in any ordinary sense. It felt more like a religious ceremony, and I wrote that night that no reporter who had not been present could fully understand the grip this man had on ordinary Germans.
Delmer reported from Berlin in early 1933 and wrote accounts of Hitler's public appearances for British readers during the weeks immediately following Hitler's appointment as Chancellor on January 30th, 1933.
FactsthatCrunchThe Reichstag Passes the Enabling Act
On March 23rd, 1933, the German parliament passed the Enabling Act, which gave Adolf Hitler the power to make laws without the approval of parliament. The act needed a two-thirds majority to pass, and many opposing members had already been arrested or kept out of the building by Nazi stormtroopers. This law effectively ended democratic government in Germany and gave Hitler full control over the country.
FactsthatCrunchFactsthatCrunchMilitarism is the belief that a country should keep a strong military and be ready to use it aggressively to gain power or protect national interests. Governments driven by militarism often spend heavily on weapons and armies. They treat military strength as a key measure of national greatness.
"Germany's militarism in the 1930s was clear when Adolf Hitler began rebuilding the German army after March 16th, 1935, in open violation of the Treaty of Versailles."
FactsthatCrunchGive me four years time, and you will not recognize Germany.
Hitler used this kind of direct promise to convince ordinary Germans that he could restore national pride and economic strength, which helped him build broad public support in the early years of his rule.
FactsthatCrunchWe are socialists, we are enemies of today's capitalistic economic system for the exploitation of the economically weak, with its unfair salaries, with its unseemly evaluation of a human being according to wealth and property instead of responsibility and performance.
Adolf Hitler, speech to the National Socialist German Workers' Party, May 1st, 1927
Hitler gave this speech during a period when the Nazi Party was still a fringe group struggling to win broad support across Germany. The quote shows how Hitler used the language of economic fairness and worker grievance to attract voters who felt left behind by the existing system. Historians treat speeches like this one as key primary sources because they reveal the promises Hitler made before he gained full power.
FactsthatCrunchIn what order did these steps happen during Hitler's rise to power?
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Black & whiteColourAppeasement Before World War II
Through the mid-1930s, Britain and France repeatedly chose to give Hitler what he demanded rather than risk another war, believing each concession would be the last. That pattern sent a clear signal about how far a determined aggressor could go before anyone would actually push back.
FactsthatCrunchI sat in the front row of the gallery in the House of Commons on the 30th of September, 1938, and watched Neville Chamberlain wave the paper he had brought back from Munich. The chamber rang with cheers that I found deeply unsettling. I had reported on Hitler's demands at Berchtesgaden only weeks before, and I knew the Czechs had been given no seat at the table when their own land was handed away. I left Westminster that evening certain that what the crowd called peace was only a short delay.
Cowles described the scene in London on September 30th, 1938, the day Chamberlain returned from Munich, in her 1941 memoir "Looking for Trouble."
FactsthatCrunchFactsthatCrunchLebensraum was a political idea that Germany needed to conquer new territory to the east in order to give its people enough land, food, and resources to survive and grow.
"Hitler used the idea of Lebensraum to justify invading Poland and pushing into the Soviet Union, claiming that Germans needed the land and farmland those countries held."
FactsthatCrunchSome historians argue that Neville Chamberlain's policy of appeasement was a reasonable response to genuine constraints, not simply a failure of courage or judgment.
FactsthatCrunchBoth happened in 1938 and showed that Britain and France would not stop Hitler's expansion. Their shared pattern of allowing German aggression without military response encouraged Hitler to push further.
After Germany absorbed Austria in March 1938, Britain and France still agreed at Munich to let Hitler take the Sudetenland from Czechoslovakia just six months later.
FactsthatCrunchA policy of giving in to another country's demands in order to avoid a war or conflict. Britain and France used appeasement with Hitler in the 1930s, hoping he would stop seeking more territory after each gain.
"Britain's policy of appeasement gave Hitler confidence that he could keep expanding without facing military resistance."
A region along the edge of Czechoslovakia that had a large German-speaking population. Hitler demanded control of it in 1938, and Britain and France agreed to hand it over as part of their appeasement policy, without asking Czechoslovakia's government.
"The transfer of the Sudetenland to Germany in 1938 is one of the clearest examples of appeasement in action, because Czechoslovakia lost the territory without ever having a say."
The idea that a group of nations can protect each other by agreeing to treat an attack on one as an attack on all. Supporters believed it could stop aggressors like Hitler, but the League of Nations struggled to make it work in the 1930s.
"Many people argued that a strong collective security agreement would have done more to stop Hitler than the policy of appeasement ever did."
FactsthatCrunchHow horrible, fantastic, incredible it is that we should be digging trenches and trying on gas masks here because of a quarrel in a far-away country between people of whom we know nothing.
Neville Chamberlain, BBC radio broadcast, London, England, September 27th, 1938
British Prime Minister Neville Chamberlain made this statement three days before he signed the Munich Agreement, which gave Germany the Sudetenland region of Czechoslovakia. He described the crisis as a distant problem to justify avoiding war with Hitler. Winston Churchill responded on October 5th, 1938, telling Parliament that Britain had chosen dishonor and would get war anyway.
FactsthatCrunchChurchill told the House of Commons on October 5th, 1938 that Britain and France had suffered a "total and unmitigated defeat" by giving in to Hitler's demands for Czechoslovakia's Sudetenland region. He warned that the agreement had deeply endangered the safety of both Britain and France, not just Czechoslovakia. Churchill believed that giving ground to an aggressor without resistance only encouraged further demands, and he had called for rearmament and firm collective action throughout the 1930s.
FactsthatCrunchWhat were the key acts of appeasement that encouraged Hitler before World War II?
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FactsthatCrunchAt the Munich Conference in September 1938, Britain and France agreed to let Hitler take the Sudetenland from Czechoslovakia. Which British Prime Minister approved this agreement and returned home claiming it meant 'peace for our time'?
FactsthatCrunchCan you match each term to its correct definition about appeasement before World War II?
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