Between 1939 and 1945, somewhere between 70 and 85 million people died. That's roughly 3% of the world's population at the time. Every continent was affected. Every major power was drawn in. The war reshaped borders, created institutions, launched technologies, and established the geopolitical order that persisted for the rest of the century.
Remove it, and the 20th century becomes unrecognisable.
But first, you have to figure out how it doesn't happen, because the war didn't emerge from nothing. It grew from specific conditions, and those conditions don't simply vanish.
The problem with prevention
World War II had a chain of causes stretching back at least two decades. The Treaty of Versailles, which ended World War I, imposed punishing reparations on Germany. The Weimar Republic's hyperinflation of 1923, the Great Depression of 1929, mass unemployment, political radicalisation, the Nazi party's rise. Each link in the chain made the next more likely.
For the war not to happen, something in this chain has to break. The cleanest break is probably this: Hitler never comes to power. Maybe the Munich Putsch of 1923 actually kills him. Maybe the German conservative establishment doesn't miscalculate by thinking they can control the Nazis as a junior coalition partner. Maybe the economy recovers faster and voters don't turn to extremists.
But here's the uncomfortable truth: even without Hitler, the conditions that produced him remain. Germany is still humiliated, still economically devastated, still politically unstable. Another demagogue might have risen. Maybe less effective, maybe less genocidal, maybe less reckless in foreign policy. Or maybe worse in ways we can't predict.
For this thought experiment, let's assume the whole war simply doesn't happen. No European theatre, no Pacific theatre. The 1930s end with tension but no global conflagration.
No nuclear weapons (yet)
The Manhattan Project existed because of the war. Specifically, it existed because Albert Einstein and Leo Szilard wrote to President Roosevelt in August 1939 warning that Germany might develop an atomic bomb. Without that threat, the US government has no reason to spend $2 billion (about $28 billion adjusted for inflation) on a crash programme to split the atom before the Nazis do.
Nuclear physics still advances. The science was already well understood by 1939. Fission had been demonstrated. The theoretical possibility of a chain reaction was known. But without wartime urgency and essentially unlimited military funding, nuclear weapons arrive later. Maybe the 1950s. Maybe the 1960s. And when they arrive, they arrive in a different geopolitical context, without Hiroshima and Nagasaki as demonstration of what they can do.
This is a double-edged outcome. No Hiroshima means 200,000 people don't die in nuclear fire. But it also means the world never gets the visceral shock that made nuclear war taboo. The first use of nuclear weapons might come in a context where the horror isn't contained, where it escalates, where there's no precedent for restraint.
Europe stays imperial
World War II broke the European empires. Britain, France, the Netherlands, Belgium, and Portugal all emerged from the war financially exhausted and militarily weakened. They could no longer afford to maintain colonial possessions across Africa, Asia, and the Pacific. The wave of decolonisation that swept the 1950s and 1960s was a direct consequence.
Without the war, these empires persist. Not indefinitely. Colonial independence movements were already building before 1939. India's independence movement was decades old. Anti-colonial sentiment in Africa and Southeast Asia was growing. But the empires would have the military and economic capacity to suppress these movements for longer.
India might not gain independence until the 1960s or 1970s rather than 1947. African nations might remain under colonial rule into the 1980s. The decolonisation process, when it eventually comes, might be slower, more piecemeal, and possibly more violent as empires resist with strength they didn't have in our timeline.
No United Nations, no NATO, no EU
Every major international institution of the post-war era was a response to the war.
The United Nations was created explicitly to prevent another world war. Its predecessor, the League of Nations, had failed to prevent the one we're removing. Without a second failure to motivate reform, the League might limp along in its weakened state, ignored by major powers, unable to enforce anything. International cooperation remains ad hoc and toothless.
NATO doesn't exist because NATO was created to contain the Soviet Union in the context of post-war Europe. Without the war, the Soviet Union is still a major power but the specific confrontation that produced the Cold War doesn't develop in the same way. American troops aren't stationed in Europe. There's no Berlin Wall because there's no divided Germany.
The European Union, or more precisely its precursor the European Coal and Steel Community of 1951, was designed to make war between France and Germany economically impossible by intertwining their industrial bases. No war, no motivation. European integration doesn't happen, or happens much later, driven by economic logic rather than the desperation of nations that had just destroyed each other twice in thirty years.
The Holocaust doesn't happen
Six million Jews, along with Roma, disabled people, political prisoners, and others, were murdered in the Holocaust. This is the most significant non-occurrence in the thought experiment, and it's the one that makes the exercise most uncomfortable.
Without the war, the industrial machinery of the Holocaust doesn't get built. The concentration and extermination camps were products of wartime conditions: the fog of war that hid them, the wartime economy that fed them, the territorial expansion that gave the regime access to Jewish populations across Europe.
But antisemitism doesn't disappear. It was endemic in Europe long before the Nazis. Pogroms, legal discrimination, social exclusion. Without the Holocaust, antisemitism remains a mainstream prejudice in European society, unchallenged by the horror that eventually forced a reckoning. European Jews survive in larger numbers but in a society that hasn't been shocked into confronting what that prejudice can produce.
Israel likely doesn't exist, or not in the form it took. The Zionist movement predated the war, but the international support for a Jewish homeland was massively accelerated by the Holocaust. Without it, the British Mandate in Palestine continues, Jewish immigration is slower, and the political situation in the Middle East follows a different path entirely.
Technology without war
An extraordinary amount of modern technology was developed or accelerated by the war. Jet engines, radar, computers (the Colossus machines at Bletchley Park were among the first electronic programmable computers), penicillin mass production, synthetic rubber, rocket propulsion, nuclear energy. War is a horrifically effective driver of technological development because it removes the normal constraints of cost, time, and caution.
Without the war, these technologies still emerge. The underlying science existed. But they emerge more slowly, driven by civilian research budgets and commercial incentives rather than existential military need. The jet age might start in the 1960s instead of the late 1940s. Antibiotics might take another decade to reach mass production. The space race, which was built on wartime rocket technology and Cold War competition, doesn't happen in the same form.
Computing develops differently. Without Bletchley Park, ENIAC, and the other wartime computing projects, the digital revolution starts from a different point. It still happens (the mathematical foundations were laid by Turing and others regardless of the war), but the massive government investment that accelerated computing by decades doesn't materialise.
Something worse?
This is the part nobody wants to consider. The post-war settlement, for all its flaws, produced the longest period of great-power peace in modern European history. The EU, NATO, nuclear deterrence, the UN Security Council: these were built on the wreckage of the war, and they held. Imperfectly, with proxy wars and near-misses, but the major powers didn't fight each other directly for over seventy years.
Without the war, none of these stabilising structures exist. Europe in the 1950s is still a continent of rival empires, unresolved territorial disputes, and competing ideologies. The fascist movements of the 1930s, even without Hitler's particular brand of it, don't simply evaporate. Authoritarian nationalism remains a live force in Spain, Italy, Hungary, Romania, and elsewhere.
The question is whether these tensions eventually produce a different catastrophic war. Maybe in the 1950s, with better weapons. Maybe with nuclear weapons, once they arrive, used by powers that never learned what they could do.
World War II was a catastrophe that killed tens of millions and left scars that haven't fully healed eighty years later. But it also, through the sheer scale of its destruction, created the conditions and institutions that prevented something similar from happening again. Remove the war and you remove the lesson. History suggests that people who haven't learned a lesson the hard way tend to learn it eventually, and the tuition is always paid in the same currency.