COLLEVILLE-SUR-MER, France — Men who stormed Normandy’s shore 70 years ago joined world leaders Friday in paying tribute to the 150,000 Allied troops who risked and lost their lives in the D-Day landings in Nazi-occupied France, in a day of international commemorations of history’s biggest amphibious invasion.

The ceremony honored the troops and civilians who fell in mighty battles that helped bring Europe peace and unity — just as bloodshed in Ukraine is posing new challenges to European security and threatening a new East-West divide.

As the sun rose Friday over a gusty Omaha Beach, flags flew at half-staff. A US military band played Taps, while D-Day veterans from the 29th Infantry Division and serving soldiers stood at attention at exactly 6:30 a.m., the moment on June 6, 1944, when Allied troops first waded ashore.

“Twenty-nine, let’s go!” they shouted, then downed shots of Calvados, Normandy apple brandy.

Hundreds of Normandy residents and other onlookers applauded the veterans, then began forming a human chain on the beach.

World leaders and dignitaries including President Barack Obama and Queen Elizabeth II converged on Normandy to honor the more than 150,000 American, British, Canadian and other Allied D-Day troops who risked and gave their lives to defeat Adolf Hitler’s Third Reich.

The D-Day invasion was a turning point in World War II, cracking Hitler’s western front as the Soviet troops made advances in the east. Overall, at least 4,400 Allied troops were killed the first day, and many thousands more in the ensuing three-month Battle of Normandy, which brought the Allies to Paris to liberate the French capital from Nazi occupation.

Russian President Vladimir Putin also attended, invited by French President Francois Hollande in a gesture toward the 27 million Soviet citizens killed in World War II.

The D-Day commemorations are also offering a moment to try to reconcile Russia and Ukraine, and Russia and the West.

Putin is scheduled to meet German Chancellor Angela Merkel in Deauville on the Normandy coast Friday morning, after meeting Hollande and British Prime Minister David Cameron on Thursday night. Ukraine’s president-elect is also coming to Normandy, and there is hope he and Putin may meet, too.

The encounters marked the first time the isolated Russian leader has met Western leaders since pro-European protests in Kiev pushed out Ukraine’s Russian-leaning president in February and Russia annexed the Crimean Peninsula.

Back in Normandy, several thousand veterans, family members and others gathered at the Normandy American Cemetery and Memorial, with its 9,387 white marble tombstones on a bluff overlooking the site of the battle’s bloodiest fighting at Omaha Beach, the emotional centerpiece of pilgrimages to honor the men killed in Normandy.

Speaking at the site, Obama said “the tide was turned in that common struggle for freedom” on D-Day and now lives on in a new generation.

“America’s claim — our commitment to liberty, to equality, to freedom, to the inherent dignity of every human being — that claim is written in blood on these beaches, and it will endure for eternity,” Obama said in remarks prepared for delivery on a morning that dawned glorious and bright over the sacred site he called “democracy’s beachhead.”

Obama spoke from the Normandy American Cemetery and Memorial, where nearly 10,000 white marble tombstones sit on a bluff overlooking the site of the June 6, 1944, battle’s most violent fighting at Omaha Beach. He described D-Day’s violent scene in vivid terms, recalling that “by daybreak, blood soaked the water” and “thousands of rounds bit into flesh and sand.”

“We come to remember why America and our allies gave so much for the survival of liberty at its moment of maximum peril,” Obama said. “And we come to tell the story of the men and women who did it, so that it remains seared into the memory of the future world.”

Obama’s speech at the morning ceremony came after he met privately with some of the dwindling number of surviving troops who fought Adolf Hitler’s Third Reich, along with those who have served since the terrorist attacks of Sept. 11, 2001.

He told the D-Day veterans, “Your legacy is in good hands.”

This June 6, 1944, file photo shows American soldiers of the Allied Expeditionary Force securing a beachhead during initial landing operations at Normandy, France, June 6, 1944. (AP Photo/Weston Haynes, File)

This June 6, 1944, file photo shows American soldiers of the Allied Expeditionary Force securing a beachhead during initial landing operations at Normandy, France, June 6, 1944. (AP Photo/Weston Haynes, File)

Soldiers of 173rd Airborne brigade, the ceremony organizers, served as ushers, wearing maroon berets. For the ceremony, small US and French flags were placed in the ground at each grave.

Obama earlier declared June 6 a national remembrance day.

Cameron paid tribute to the relationship forged between Britain and France in the battles at Normandy.

“Just as British and French soldiers fought for victory against a common enemy on the beaches of Normandy, today France and the UK stand shoulder to shoulder against the threats of the modern world,” he said, according to a Guardian report. “We remain united against international terrorism and extremism – and in recent times our armed forces have served together in Afghanistan, Libya, Mali and elsewhere around the world.

Marissa Neitling, 30, from Lake Oswego, Oregon, places a flower on a grave in the Normandy American Cemetery and Memorial, in Colleville sur Mer, France, Wednesday June 4, 2014. (photo credit: AP Photo/Claude Paris)

Marissa Neitling, 30, from Lake Oswego, Oregon, places a flower on a grave in the Normandy American Cemetery and Memorial, in Colleville sur Mer, France, Wednesday June 4, 2014. (photo credit: AP Photo/Claude Paris)

In addition to the fallen troops, Allied bombardments killed an estimated 20,000 French civilians, and Hollande paid tribute to them Friday in Caen, which like many cities of Normandy was largely destroyed in the bombings.

France has only tentatively come to grips with the invasion’s toll on civilians. The Allied bombings — especially the deadly onslaught in Normandy during the invasion launched on D-Day — were used as a propaganda tool by the Vichy government. But historians now believe that nearly as many French civilians died in Allied air raids as Britons during the German Blitz.

“No one knew that this day would be the first of one of the most ferocious battles of France. This battle was also a battle of civilians,” Hollande said. He said Normandy’s residents “helped the victory happen. They opened their doors to the liberators.”

Ceremonies large and small are taking place across Normandy, ahead of an international summit on Friday in Ouistreham, a small port that was the site of a strategic battle on D-Day.

Today’s conflicts are also on many minds at the D-Day commemorations.

Jeffrey McIllwain, professor at the San Diego State University school of public affairs, will lay a wreath on behalf of educators who have lost students to wars in Iraq and Afghanistan — himself included.

He, like many veterans and world leaders here, is concerned about keeping the memory of D-Day alive as the number of survivors dwindles.

He brought 12 students to Normandy for a course on the lessons of D-Day.

“I make them promise to bring their grandchildren to serve as a bridge to the next generation,” he said.