On 3 September 1939, Britain declared war on Adolf Hitler's Nazi Germany.
At 11:15am that morning, Prime Minister Neville Chamberlain admitted to the nation in a sombre radio broadcast that his "long struggle to win peace" had failed.
Hours later, France issued its own ultimatum to Germany, setting in train the Second World War.
It was a conflict which lasted nearly six years and cost around 50 million lives.
The British Prime Minister's radio address was made from the Cabinet room in 10 Downing Street.
He said: "This morning the British ambassador in Berlin handed the German Government a final note stating that, unless we heard from them by 11 o'clock that they were prepared at once to withdraw their troops from Poland, a state of war would exist between us.
"I have to tell you now that no such undertaking has been received, and that consequently, this country is at war with Germany."
Mr Chamberlain continued his broadcast by saying it was "a bitter blow" that his "long struggle to win peace" had "failed".
"Yet I cannot believe that there is anything more or anything different that I could have done, and that would have been more successful," he added.
He concluded his broadcast: "Now may God bless you all and may he defend the right.
"For it is evil things that we shall be fighting against - brute force, bad faith, injustice, oppression, and persecution. And against them, I am certain that the right will prevail."
A year earlier, Mr Chamberlain returned to London from talks in Munich clutching an agreement signed by Hitler that he said meant "peace for our time".
But the German leader continued his aggression in Europe, culminating in the invasion of Poland.
Britain went on to suffer an estimated 400,000 military and civilian casualties.
Cover image: De Haviland Mosquito FB.VIs of 248 Squadron attacking a German 'M' Class minesweeper and two trawler-type auxiliaries in France 1944 (Picture: MOD).