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.
Oana Lungescu, BBC News, Berlin
With no more surviving WWI veterans on either side of the river Rhine, today's generation of leaders want to give this anniversary a new meaning as a celebration of friendship.
German commentators describe Chancellor Merkel's trip to Paris as a symbolic gesture of reconciliation. Her predecessor, Gerhard Schroeder, had turned down a similar invitation, arguing he had to be somewhere else.
There's a broader attempt to restart the Franco-German axis, which for decades has driven European integration. But there's no clear agreement on what more they can do together.
France has come up with some bold ideas, including a single minister for bilateral relations sitting in both cabinets. But Germany's proving reluctant, especially when it comes to closer economic and financial co-operation.
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.
WORLD WAR I
28 Jul 1914: Austria-Hungary declares war on Serbia
1914-17: Major world powers enter war, stalemate of trench warfare on Western front
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.
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