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The Cold War did not begin with a single declaration. It grew out of the very alliance that had been built to defeat Nazi Germany. Between 1941 and 1945 the United States, the Soviet Union and Britain cooperated as the Grand Alliance, yet by the time that alliance had delivered victory in Europe it had already begun to fracture. Ideological distrust, competing visions of post-war Europe and the arrival of atomic weapons pushed the former partners apart. By March 1946 Winston Churchill could describe an "Iron Curtain" dividing the continent, and a new global confrontation between capitalism and communism was under way.
This lesson covers the origins of the Cold War up to the end of 1945 and the early months of 1946. It examines the ideological gulf between the superpowers, the wartime conferences at Tehran, Yalta and Potsdam, and the mutual suspicion that the Manhattan Project and Soviet actions in Eastern Europe were already generating. For Edexcel 1HI0 Paper 2, this is the foundation period of the Period Study: every later crisis — Berlin, Cuba, Afghanistan — can be traced back to the choices made by Stalin, Roosevelt, Truman, Churchill and Attlee in these years.
The United States and the Soviet Union were not simply rival states. They represented competing systems that each side believed was universal and correct. Understanding this ideological gulf is essential: it explains why every practical disagreement — over Poland, reparations, Germany — escalated into something larger.
The United States was a capitalist liberal democracy. Its political system rested on competitive multi-party elections, an elected president and congress, and a written constitution protecting civil liberties. Its economy was based on private ownership, free enterprise and market competition. American leaders believed that freedom of trade and freedom of speech went together, and that other nations would be more peaceful if they adopted similar systems. Roosevelt's 1941 "Four Freedoms" speech — freedom of speech, of worship, from want, from fear — expressed this worldview clearly.
The Soviet Union was a one-party communist state shaped by Marxism-Leninism. Only the Communist Party was permitted. Industry, banking and most agriculture were owned by the state. Stalin's five-year plans set production targets from the centre. Civil liberties in the Western sense did not exist: censorship was routine and the NKVD (secret police) suppressed opposition. Soviet ideology held that capitalism would eventually collapse and that communism would spread across the world; the USSR saw itself as the leader of that process.
| Feature | United States | Soviet Union |
|---|---|---|
| Political system | Multi-party liberal democracy | One-party communist state |
| Economy | Capitalist, privately owned | Command economy, state owned |
| Leadership 1945 | Roosevelt, then Truman | Stalin |
| Core belief | Individual freedom, free trade | Class struggle, collective ownership |
| View of the other | Aggressive, expansionist | Exploitative, imperialist |
These were not small differences. Each side genuinely believed the other's system was a threat to its own survival. When disagreements arose over practical questions — who should govern Poland, how Germany should be treated, how much reparations the USSR should receive — they were filtered through this ideological suspicion. A disagreement that two liberal democracies might have settled became, between Washington and Moscow, evidence of bad faith.
The Grand Alliance was an alliance of necessity, not of friendship. It came together in 1941 only because Germany attacked the Soviet Union and Japan attacked the United States.
In June 1941 Hitler launched Operation Barbarossa, invading the Soviet Union. Stalin, who had signed the Nazi-Soviet Pact in 1939, suddenly found himself fighting for survival against the Wehrmacht. Churchill, a lifelong anti-communist, immediately offered British support: as he put it, "If Hitler invaded Hell I would make at least a favourable reference to the Devil in the House of Commons." In December 1941 Japan attacked Pearl Harbor and Germany declared war on the United States, bringing America formally into the war in both theatres.
The three powers — Britain, the USA and the USSR — now shared an enemy in Nazi Germany. They agreed broad war aims in the 1941 Atlantic Charter (Britain and the USA) and extended these to the USSR through Lend-Lease aid. Yet tensions existed from the start. Stalin repeatedly pressed for a "Second Front" — an Anglo-American invasion of Western Europe — to relieve pressure on the Red Army. Churchill and Roosevelt delayed this until June 1944 (D-Day), partly for practical reasons and partly, Stalin suspected, to let the USSR bear the heaviest costs. The Soviet Union lost an estimated 27 million people in the war, a scale of loss that shaped Stalin's determination to secure a friendly buffer zone in Eastern Europe at its conclusion.
The first of the three Big Three conferences took place in Tehran, Iran, from 28 November to 1 December 1943. Roosevelt, Churchill and Stalin met in person for the first time.
The main decisions at Tehran were:
Tehran set the pattern for wartime cooperation: the Big Three could agree on practical military questions when they shared an immediate interest, but political questions about post-war Europe were left vague. Roosevelt believed personal relationships — what he called his ability to "handle Stalin" — would keep the alliance together.
By the time the Big Three met again in February 1945 at Yalta, in the Crimea, the war in Europe was almost won. The Red Army had driven through Eastern Europe and was close to Berlin; British and American forces had crossed the Rhine. The conference was held at a Tsarist palace on the Black Sea.
Yalta agreements included:
Yalta looked, on the surface, like a major success. But two problems were embedded in it. First, the phrase "free elections" meant different things to Roosevelt and Stalin: the Americans expected genuinely competitive votes, while Stalin intended communist-dominated governments loyal to Moscow. Second, with Red Army soldiers already occupying Eastern Europe, Stalin held the practical power on the ground. He could sign agreements in February and enforce his own version of them in practice.
The final Big Three conference took place at Potsdam, just outside Berlin, from 17 July to 2 August 1945. Germany had surrendered in May. The mood was very different from Yalta.
Two of the three leaders had changed. Roosevelt had died in April 1945 and was replaced by Harry S. Truman, a far less trusting figure who believed Roosevelt had given away too much at Yalta. Churchill was replaced mid-conference by Clement Attlee, who won the British general election of July 1945. Only Stalin remained.
Crucially, on 16 July 1945 — the day before Potsdam opened — the United States successfully tested the first atomic bomb at Alamogordo, New Mexico. Truman now possessed a weapon of unprecedented destructive power. He mentioned it to Stalin in general terms during the conference. Stalin, whose intelligence services had already told him about the Manhattan Project, appeared unconcerned and simply encouraged the Americans to use it against Japan.
Potsdam agreements and disputes included:
Potsdam did not produce the warmth of Tehran or the apparent agreement of Yalta. Truman was suspicious; Stalin was determined to hold what the Red Army had taken. The atomic monopoly of the United States changed the psychological balance of the alliance.
flowchart TD
A[Grand Alliance 1941] --> B[Tehran Nov 1943]
B --> C[Yalta Feb 1945]
C --> D[Germany surrenders May 1945]
D --> E[Potsdam July 1945]
E --> F[Atomic bombs Aug 1945]
F --> G[Eastern Europe falls under Soviet control 1945-46]
G --> H[Iron Curtain speech March 1946]
Between August 1945 and March 1946 mutual suspicion hardened. The atomic bombings of Hiroshima (6 August 1945) and Nagasaki (9 August 1945) ended the war against Japan but also raised Soviet fears that the United States might one day use such weapons as political leverage. Stalin ordered the Soviet atomic programme to be accelerated.
In Eastern Europe, the promised free elections did not materialise in the Western sense. Communist-led coalition governments took power in Poland, Bulgaria, Romania and Hungary, often with the Red Army in the background. Non-communist opposition figures were harassed, arrested or forced into exile. To American observers, Stalin was tearing up the Yalta agreement; to Stalin, he was securing the buffer zone the USSR had paid for in blood.
On 5 March 1946 Winston Churchill — no longer prime minister but invited by Truman — delivered a speech at Westminster College in Fulton, Missouri. He declared:
"From Stettin in the Baltic to Trieste in the Adriatic, an iron curtain has descended across the Continent."
Churchill called for a close Anglo-American alliance to contain Soviet expansion. Stalin responded furiously, comparing Churchill to Hitler and accusing him of warmongering. The phrase "Iron Curtain" captured the new reality: Europe was dividing into two blocs, and the wartime alliance was effectively over.
This content is aligned with the Edexcel GCSE History (1HI0) Paper 2 Period Study specification.