But he added: "The commemoration should be cause for reflection about the importance of freedom. Alas, too many European leaders today have embraced moral compromise, and convince themselves that short-term economic opportunism trumps liberty."
The roll call of VIPs includes the Queen - at 88, making a rare foreign trip with Prince Philip - as well as Obama, Putin, Canadian Prime Minister Stephen Harper, German Chancellor Angela Merkel, President Francois Hollande of France and Australia's Prime Minister, Tony Abbott.
The greatest amphibious assault in history, D-Day opened up a western front in continental Europe, progressively squeezing Nazi Germany between US and British forces on one side and Soviet forces on the other.
Masked by British intelligence, who fooled the Germans into believing the long-expected operation was aimed farther north in the Pas-de-Calais region and Norway, the allies landed 156,000 men in a single day.
Five days later, 326,000 American, British, Canadian, Polish and French troops had been established on the bridgehead and the Battle of Normandy, which saw bloody tank combat in hedgerowed fields and hand-to-hand fighting in shattered villages, was under way.
The scars of this epic chapter remain today.
At Gold beach, the huge concrete structures of the Mulberry Harbour, an ingenious artificial port that eventually landed 2 million men, half a million vehicles and 4 million tonnes of supplies, become eerily exposed at low tide, tinged green by seaweed and eroded by decades of surf.
Farther west, gigantic craters still pit the Pointe du Hoc, a 30m cliff that US Rangers scaled using ladders supplied by the London Fire Brigade to wipe out German defences.
Nearby, within earshot of the waves, lies the vast US war cemetery at Colleville-sur-Mer, its emerald lawns and towering pines the final resting place for 9387 Americans who died in action.
Few visitors can be unmoved by the sheer size of Colleville and the emotions it evokes of courage, sacrifice and lost youth.
Bequeathed by France in perpetuity, the cemetery was the starting and endpoint for Steven Spielberg's Oscar-winning Saving Private Ryan. On Friday, it will provide Obama with a backdrop to drive home a message to Europeans of their historic debt to America and their dependence, even today, on the US military shield.
Six months ago, US-European relations were at a very low ebb, damaged by revelations of mass eavesdropping on European citizens by the US National Security Agency.
Today, Putin's grab of Crimea and meddling in eastern Ukraine have prompted the mood to change to relief that the Americans and Nato are there as a bulwark against Russian adventurism.
The situation in Ukraine remains volatile and suspicions of the Kremlin run deep - so a meeting on the sidelines in Normandy could do much to rebuild trust or set a tone.
Hollande, who invited Putin personally to the ceremony, will meet the Russian strongman on Thursday on the eve of the anniversary. On Friday, heads of state will gather for a lunch at Benouville before heading for the main commemoration at Ouistreham.
"Putin is a man who is a big poker player," said Bertrand Badie, a professor at the top Paris school for political science, Sciences-Po. "He could use this meeting to pass on a message, to say something. But as for negotiation, no."
"Western leaders certainly will see Mr Putin," said Klaus Segbers of the Centre for Global Politics at Berlin's Free University. He said the usefulness may lie in reaffirming the West's firm line.
"Russia's policy over the last couple of weeks is more careful than before. The combined costs of capital fleeing the country, stopped investment projects and infrastructure construction on Crimea are looming. If this current restraint of the Russian President can be prolonged, it would in itself be a lot."
