Services of commemoration are being held around the world to mark Armistice Day, the end of World War I in 1918.
At a speech at Arlington National Cemetery, US President Barack Obama said no tribute could match the service and sacrifice of the armed forces.
Earlier, Angela Merkel became the first German chancellor to mark the day by attending French events in Paris.
In London, the Queen, politicians and British Armed Forces chiefs recalled the passing of the WWI generation.
Mr Obama laid a wreath at the Tomb of the Unknowns at Arlington National Cemetery, where America's war dead are buried.
He praised the "extraordinary bravery and service" of the armed forces past and present.
"To the veterans, the fallen and their families - there is no tribute, no commemoration, no praise that can truly match the magnitude of your service and your sacrifice," he said.
Mr Obama said the armed forces had been "doing right by America" for generations and promised them his continuing support.
"As long as I am commander in chief, I am going to do right by them. America will not let you down, we will take care of our own," he said.
In Paris, French President Nicolas Sarkozy and the German chancellor rekindled the flame on the Tomb of the Unknown Soldier at the foot of the Arc de Triomphe.
The two leaders also observed a silence, flanked by soldiers from a Franco-German brigade and officers from both countries' armed forces.
They were reunited just two days after both played a prominent role in the events marking the 20th anniversary of the fall of the Berlin Wall.
The BBC's Emma Jane Kirby in Paris says the two leaders standing side by side at the tomb, while the national flags of both countries fluttered in the breeze, was a powerful and poignant symbol of reconciliation.
Speaking after the formal ceremonies, Mr Sarkozy told Mrs Merkel: "Your presence among us on this November 11th is a gesture of exceptional friendship - every French person knows how significant it is."
Mrs Merkel responded: "Both dates, the end of the First World War, and the day of the fall of the Berlin Wall, remind us that we must always fight for the invaluable goods of peace and freedom, that we need to defend our values, of democracy and human rights, and that we keep working for European solidarity and partnership with America. That is our task."
Mr Sarkozy also said 11 November should not be a day to celebrate one country's win over another, and that it should be remembered that German children had cried for their dead fathers in exactly the same way as French children had.
Mrs Merkel said her country readily accepted the French gift of friendship and she thanked history for bringing the two nations together.
Guns fell silent
The memorial service in London, at Westminster Abbey, was being held following the deaths this year of the final three veterans of the war living in the UK.
The Queen led the country in observing a two-minute silence at 1100 GMT for the "passing of a generation".
The last three surviving WWI veterans in the UK died this year - Bill Stone died at age 108 in January, followed by both Henry Allingham, 113, and Harry Patch, 111, in July.
At the main UK military base in Afghanistan, Camp Bastion in Helmand province, a padre led prayers for the dead and injured in all conflicts since World War I.
The BBC's Adam Mynott reported that a heavy gun was fired, and for two minutes those who could stood still in silence to pay respects for those killed in war.
He added that at the forefront of people's minds were the 233 British service personnel who had died in Afghanistan since military operations began in 2001.
Australia's east coast, which is 11 hours ahead of GMT, was one of the first places to commemorate the 11th hour of the 11th day of the 11th month - the time the guns of World War I fell silent.
The names of five Australian soldiers killed in Afghanistan since Remembrance Day last year were being added to the roll of honour at the Australian War Memorial in the capital, Canberra, Australian television reported.
Troop levels in Afghanistan will be on the agenda in Washington later on Wednesday when President Obama meets his national security team.
An exhaustive review of US strategy in the country, including troop numbers, appears to be drawing to a close with unofficial reports suggesting Gen Stanley McChrystal, the US commander, will get somewhat fewer than the 40,000 extra soldiers he has been asking for.