World War II (1939–1945) Explained Simply

World War II lasted from 1939 to 1945 and remains, to this day, the deadliest and most consequential war in human history. Estimates put the global death toll at 60 to 70 million - soldiers and, on a scale never seen before, civilians. It was fought simultaneously on several continents, from Western Europe through North Africa to the Pacific, and it is inseparably linked to one of the greatest crimes in history: the Holocaust, the systematic murder of millions of people by Nazi Germany. To understand this war, you have to start with its causes - and those trace all the way back to the final days of World War I.

One important root of World War II lies in the Treaty of Versailles of 1919, which had ended World War I. Many Germans experienced the war guilt clause it imposed, the heavy reparations, and the loss of territory as a deep humiliation. Far-right forces deliberately exploited this bitterness - for example with the claim that the German army had never actually been defeated on the battlefield but had instead been "stabbed in the back" by Social Democrats and Jews (the so-called "stab-in-the-back myth"). This claim had no basis in the actual military facts, but it was politically very effective, because it offered simple scapegoats for a complicated defeat.

The situation grew worse with the Great Depression, which began in the United States in 1929 and quickly spread around the world. In Germany, unemployment climbed to over six million people by 1932. Economic despair, combined with the sense that the democratic parties of the Weimar Republic no longer had answers, pushed many voters toward the political extremes - above all toward the NSDAP under Adolf Hitler, which promised simple explanations and a radical fresh start.

On January 30, 1933, President Paul von Hindenburg appointed Adolf Hitler as chancellor - entirely within the bounds of the constitution. Within a few months, though, Hitler turned that position into a dictatorship: after the Reichstag fire in February 1933, basic civil rights were suspended, and in March 1933 the NSDAP forced through the so-called Enabling Act, which let the government pass laws without parliament. Other parties were banned or dissolved themselves under pressure, trade unions were smashed, and the press was brought under Nazi control. Within a matter of months, a democracy had turned into a totalitarian dictatorship built around a single leader.

A central pillar of Nazi ideology was a dehumanizing racial doctrine: the Nazis sorted people into "valuable" and "inferior" groups and placed the so-called "Aryan race" at the top of an invented hierarchy. At the very center of their hatred stood a radical antisemitism that blamed Jews as scapegoats for virtually every problem in Germany. Alongside this came the idea of "Lebensraum in the East": Hitler and the Nazi leadership wanted to conquer the Soviet Union, subjugate, expel, or murder the people who lived there, and resettle the land with German colonists. This intent to conquer and exterminate was built into Nazi planning from the very start - it was not merely something that emerged later, as the war unfolded.

Starting in the mid-1930s, Hitler pursued his goals step by step against surprisingly weak international resistance. In 1936 he had the Rhineland remilitarized, even though the Treaty of Versailles explicitly forbade it. In 1938 he forced the "Anschluss," the annexation of Austria into the German Reich. That same year, he demanded the mostly German-speaking Sudetenland from Czechoslovakia; in the Munich Agreement of September 1938, Britain and France gave in to this pressure to avoid war - a policy that became known as "appeasement." The peace it bought didn't last: by March 1939, Hitler had occupied the rest of Czechoslovakia too, breaking the Munich Agreement himself. By this point, at the latest, Western leaders understood that Hitler's ambitions would not stop at "German" territory.

Hitler took power in 1933; the war in Europe began with the invasion of Poland in 1939; and in 1941, with the German attack on the Soviet Union and Japan's attack on Pearl Harbor, it finally became a truly global war.

On September 1, 1939, the German Wehrmacht invaded Poland without a declaration of war - this is considered the beginning of World War II. Shortly before, on August 23, 1939, Germany and the Soviet Union had surprised the world with the Hitler-Stalin Pact, a non-aggression treaty with a secret protocol that divided Eastern Europe into German and Soviet spheres of influence - Poland was to be split between the two powers. Because Britain and France had already pledged to defend Poland, they declared war on Germany on September 3, 1939. The Polish army was no match for the German "Blitzkrieg" tactics - fast, coordinated attacks combining tanks and air power - and was defeated within a few weeks, with the Soviet Union invading from the east on September 17 to complete the conquest.

German success continued over the following months: Denmark and Norway were occupied in 1940, and in May 1940 the Wehrmacht overran the Netherlands, Belgium, and finally France, which surrendered as early as June 1940 - a defeat many had considered unthinkable given the experience of World War I. Only Britain, under Prime Minister Winston Churchill, kept fighting, winning the "Battle of Britain" against the German air force in the summer and fall of 1940 and thereby preventing a planned German invasion of the island.

A decisive turning point came on June 22, 1941: on that day, Germany attacked the Soviet Union despite the non-aggression pact - "Operation Barbarossa," the largest military operation in history up to that point. German troops initially pushed deep into Soviet territory, but the hoped-for quick victory never came. The brutal Russian winter, vast distances, and stubborn Soviet resistance ground the Wehrmacht down; the Battle of Stalingrad in 1942–43, in which an entire German army was surrounded and destroyed, is regarded by many historians as the military turning point of the entire war in Europe. From then on, the Wehrmacht was mostly in retreat on the Eastern Front.

At almost the same time, the war expanded into a truly global conflict: on December 7, 1941, Japan - which had already been waging a brutal war against China since 1937 and was allied with Germany and Italy through the Tripartite Pact - attacked the American fleet at Pearl Harbor, Hawaii, without a declaration of war. The United States declared war on Japan in response; a few days later, Hitler declared war on the United States, formally bringing America into the war against Germany as well. From this point on, the Allies (including the US, Britain, and the Soviet Union) and the Axis powers (Germany, Italy, and Japan) faced each other on a truly global scale.

While the war raged, the Nazi regime was simultaneously carrying out the systematic murder of people on a scale never seen before: the Holocaust, also known by its Hebrew name, the Shoah. Jews in Germany had already been progressively stripped of their rights since 1933, notably through the Nuremberg Laws of 1935, which revoked their German citizenship and banned marriage with "Aryans." After the German invasion of the Soviet Union in 1941, so-called Einsatzgruppen - mobile killing squads - began systematically shooting Jewish civilians in the occupied territories, often on the spot, in mass shootings such as the one at Babyn Yar near Kyiv, where more than 33,000 people were murdered over two days in September 1941. From 1942 onward, the Nazi leadership shifted to an even more systematic, industrially organized form of mass murder: at the Wannsee Conference on January 20, 1942, senior Nazi officials in Berlin coordinated, with chilling bureaucratic precision, the deportation and murder of Europe's Jews - what they cynically called the "Final Solution to the Jewish Question." In purpose-built death camps such as Auschwitz-Birkenau, Treblinka, Sobibor, Belzec, Chelmno, and Majdanek, people were transported by train from across Europe and murdered in gas chambers, their bodies then burned.

In total, the Nazis and their collaborators murdered around six million European Jews - roughly two-thirds of Europe's pre-war Jewish population, including an estimated 1.5 million children. Hundreds of thousands of Roma and Sinti were also murdered, in what is known as the Porajmos; along with political opponents of the regime, people with disabilities under the so-called "euthanasia" murder program known as Aktion T4, gay men, Jehovah's Witnesses, and millions of Soviet prisoners of war who were deliberately killed through starvation and neglect. The Holocaust was not a byproduct of the war - it was a meticulously planned, bureaucratically organized crime that developed its own administrative machinery, transport schedules, and methods of industrialized killing. It stands as the most thoroughly documented crime against humanity in history, and it remains a central point of reference today for remembrance culture, memorial sites, and the enduring question of how a modern, culturally advanced society could become capable of such a crime.

💬 Mia asks Grandpa Theo

In the west, the Allies opened a second major front against Nazi Germany on June 6, 1944, with the landings in Normandy (D-Day), while the Red Army kept advancing from the east. In the months that followed, German forces fell onto the defensive on every front. Soviet troops encircled Berlin in April 1945; on April 30, 1945, Adolf Hitler took his own life in his bunker. The Wehrmacht surrendered unconditionally - first on May 7 in Reims and, because Soviet leaders insisted on a separate signing, a second time overnight on May 9 in Berlin. That is why the West marks May 8 and Russia marks May 9 as "Victory Day" to this day - the war in Europe was over.

In the Pacific, the US kept fighting Japan into the summer of 1945, as Japan refused to surrender despite its hopeless position. To force Japan to give up and, according to the official American justification, to avoid a ground invasion expected to cause extremely heavy casualties, the United States dropped an atomic bomb on Hiroshima on August 6, 1945, and a second one on Nagasaki three days later, on August 9, 1945. The two bombs killed an estimated 200,000 people in total, immediately and through the effects of radiation, the overwhelming majority of them civilians - to this day, these remain the only nuclear weapons ever used in war. On September 2, 1945, Japan signed its instrument of surrender, bringing the war in Asia, and World War II as a whole, to an end.

The consequences of the war shaped the world for decades. Germany was divided into four Allied occupation zones, out of which two German states emerged in 1949 - the Federal Republic of Germany in the west and the German Democratic Republic in the east - and Europe as a whole increasingly split into a US-aligned west and a Soviet-dominated east. On October 24, 1945, the United Nations was founded to prevent future wars through international cooperation - also a lesson drawn from the failure of the League of Nations after World War I. At the Nuremberg trials of 1945–46, leading Nazi officials were, for the first time in history, held accountable before an international tribunal for war crimes and crimes against humanity - establishing a principle that still holds in international law today: individuals, not just states, can be held criminally responsible for such acts. At almost the same time, the Cold War began, the decades-long rivalry between the former wartime allies, the US and the Soviet Union, that would shape the second half of the 20th century.