The friendship and cooperation of the wartime conferences ended.

At this conference the Big Three confirmed some of the decisions made at Yalta:

• Germany was split into four zones, each run by an Allied army.
  
• Germany was to be disarmed, and war criminals put on trial.
  
• Germany was to pay some reparations.
  

• Stalin wanted to see Germany permanently crippled. Truman did not want to repeat the mistakes made at Versailles in 1919.
  
• Stalin demanded huge reparations from Germany. Truman resisted this, because of the mistakes of 1919.
  
• Truman and Attlee became dissatisfied with Soviet intentions in eastern Europe and were highly suspicious of the pro-Soviet governments that Stalin was imposing. Truman felt that he would have to 'get tough' with Stalin.

On July 16 the Americans successfully tested an atomic bomb in New Mexico. At the start of the Potsdam Conference Truman told Stalin about this, but gave him very few details. However, Stalin had known about the bomb from Soviet spies in the USA, and he knew how powerful it was.

The fact that the Americans had kept the bomb secret from their Soviet Allies filled Stalin with fear and mistrust.

Because they had the atomic bomb the Americans had less need of Soviet help to defeat Japan. After atomic bombs were dropped on Hiroshima and Nagasaki in August 1945, Japan surrendered and the Second World War was over. On August 8, 1945, the day before Nagasaki, the Soviet Union declared war on Japan. It was no more than a gesture.

At the same time Truman discontinued American supplies to the Soviet Union, and a huge loan to the Russians was turned down by the American Congress, which felt that now that the war was over it was under no obligation to help the Soviet Union. There was also a strong anti-Communist feeling in Congress.

The Grand Alliance had come apart. The Cold War had begun in earnest.