On the eve of World War I, Britain's foreign minister, Edward Grey, observed: "The lamps are going out all over Europe. We shall not see them lit again in our lifetime."
As Britain and Commonwealth countries mark the centenary of the declaration of war on Germany on Monday, London will switch off the lights at landmarks such as Trafalgar Square, the Houses of Parliament and St Paul's Cathedral for an hour in the evening in tribute. Other cities around Britain will do the same.
More than one million soldiers from Britain and its former empire died in the conflict. New Zealand lost 2 percent of its total wartime population.
"Most of us will have ancestors who fought, many from what is now the Commonwealth ... and every single one of us is indebted to that generation because their legacy is our liberty," said Prime Minister David Cameron at London's Imperial War Museum last month.
"It wasn't just Britons who secured Allied victory. It was Indians, Canadians - even a Chinese Labour Corps," added Cameron, who said six of his own relatives died in the fighting.
Soldiers from the former British empire including Australia, New Zealand, Canada, India and troops in the Middle East and North Africa all fought in the war under the banner of the British Army.
Of the roughly 9 million who served, 1.1 million died, according to the Commonwealth War Graves Commission. About 74,000 were from India, 65,000 from Canada and 62,000 from Australia.
But most casualties were from the United Kingdom, including Ireland and small dominions with 888,230 men, many of them still teenagers, killed - more than double the number of its casualties in World War Two.
Britain's royal family and senior politicians from Britain, Ireland, Germany, Belgium and the Commonwealth will attend remembrance services in London and Glasgow and an event in Belgium to commemorate the centenary on Monday.
Candles at an official service in London's Westminster Abbey will go out one by one until only a burning oil lamp remains at the Grave of the Unknown Warrior. At 2200 GMT, the lamp will be extinguished, marking the exact time the British Empire joined the war. In Trafalgar Square, one single light will shine from an old police box.
Hundreds of organisations, companies, landmarks and local authorities will also participate in the "Lights Out" campaign - organised by 14-18 NOW, the official cultural programme for the centenary, by leaving a single light on for shared reflection.