When Britain declared war on Germany on 3 September 1939 Nazi leader Adolf Hitler was stunned.

His previous dealings with the British Prime Minister Neville Chamberlain had led him to believe the British would never dare risk war with Germany.

The Nazi dictator turned to his foreign minister, Joachim Von Ribbentrop and said simply: "What now?"

As historian at the Imperial War Museum, Terry Charman, told BBC News Online: "Hitler wanted a war but he didn't want the war he got."

Two days earlier, on 1 September, Hitler had unleashed his massed tanks, planes and infantry on Germany's weaker neighbour Poland.

It was the move that finally sparked World War II - one of the largest conflicts in human history.

While German troops tore across the border, using faked Polish attacks on the German radio transmitter at Gleiwitz as their excuse for invasion, divisions of SS troops prepared to follow behind the regular army.

As Polish towns and cities burned under the ferocious attacks of the Nazi dive-bombers, Poland's allies Britain and France hesitated.

Memories of World War I

The 3,000 French tanks and their 75 infantry divisions remained silent on Germany's western border, while in Britain children were being bundled into trains, taking them away from their parents and homes in the cities to the countryside - considered out of the reach of Nazi bombers.

The scars of World War I laid heavy on the two nations, but however reluctant their governments were to go to war against Germany once more - Hitler's cynical invasion of Poland ultimately forced their hands.

With his policy of appeasement in tatters, the British Prime Minister, Sir Neville Chamberlain teetered on the brink of declaring war. It was the last thing he wanted.

But in the face of a revolt from members of his Cabinet and a growing feeling in the country that Hitler must be tackled he had little choice.

Mr Charman told News Online: "The public mood was such that Chamberlain would have probably been lynched if he had tried publicly to get out of his obligations to Poland."

France in the front line

If the British government was unwilling to go to war so were Poland's allies, the French.

With the British Army capable of putting just two divisions in the field the French were painfully aware of their role. As Mr Charman says: "If any fighting was going to be done on the Allied side in September 1939 it was going to be done by the French."

Meanwhile in Britain the blackout had already begun, the fleet had been mobilised, Winston Churchill had been offered a post in the Cabinet and large public gatherings had been banned.

Gas masks were being distributed and the public had already been given information on how to prepare for air-raids. The wireless offered advice on how to make the most of tinned foods - fresh food would soon be in short supply.

With German attacks on Polish cities becoming increasingly violent, on Sunday 3 September Britain finally presented Germany with an ultimatum.

When the British Ambassador to Germany took the document to the German foreign ministry at 9 o'clock that Sunday morning the Nazi Foreign Minister Von Ribbentrop was absent.

The ultimatum

The ultimatum, which gave Germany two hours to withdraw its troops from Poland, was handed in to a lowly interpreter.

"Not for a moment did the Nazis give the document any serious consideration," says Mr Charman.

With no positive response from the Germans it was left to Chamberlain to announce to the British people over the radio that as of 11 o'clock 3 September 1939, the British Empire was at war with Germany.

The French declaration of war followed five hours later.

Australia, New Zealand and India all declared war on Germany on the same day - the other major nations of the British empire, South Africa and Canada, had declared war within the week.

But Ireland, America and Japan all declared their neutrality.

US neutral

In one of his famous fireside chats the American President Franklin Roosevelt made plain his country's intention to keep out of the war.

But though he refused to join the fight against the Nazis immediately, he told the American people that although the US would remain neutral in deed the people did not have to remain neutral in their hearts.

Within minutes of Chamberlain's declaration of war, air raid sirens sounded across London. The public, for the first time, rushed for the safety of the shelters, waiting for the all clear to sound.

Luckily it was a false alarm. But the casualties in Poland were real enough.

Already thousands of soldiers and civilians had died at the hands of the Nazi war machine, but Poland was just the beginning.

By the time the war ended in victory for the Allies in 1945 nearly 50 million men, women and children from across five continents had died.