President Donald Trump is attending the Bastille Day parade in Paris, where US and French troops are marching together down the Champs-Élysées.
The parade – marking 100 years since the Americans entered World War One – also features horses, planes and helicopters.
French President Emmanuel Macron, standing in a military jeep, inspected the troops lined up on the avenue.
Earlier Mr Trump said “the fight for freedom” united the US and France.
Mr Macron travels to Nice later to join commemorations there of last year’s Bastille Day terrorist attack, in which a Tunisian-born man drove a huge lorry into a celebrating crowd on the beachfront, killing 86 people.
On Thursday, Mr Trump suggested he could review his position on climate change, after Mr Macron argued in defence of the 2015 Paris accord.
“Something could happen with respect to the Paris accord,” Mr Trump said. “We’ll see what happens.”
Last month he had said the US would withdraw from the Paris accord, citing moves to negotiate a new “fair” deal that would not disadvantage US businesses.
On Thursday, US First Lady Melania Trump toured Notre Dame cathedral with her French counterpart Brigitte Macron, and prayed before a statue of the Virgin Mary.
President Trump told reporters “this is a wonderful national celebration”. “Our two nations are forever joined together by the spirit of revolution and the fight for freedom.”
France remains under a state of emergency, following a spate of terror attacks by jihadists.