Overnight in London, an honor guard stood vigil at the grave of the Unknown Warrior.
On Friday morning, across Great Britain, citizens observed a moment of silence.
And midday Friday, at a quiet field in northern France, British and French leaders paused amid a political crisis for a brief period of solidarity.
They gathered together at the site of the Battle of the Somme, 100 years after the bloodiest day in British military history, to honor the dead.
That costly World War I battle in France stretched on for months, but it's the horror of the first day that looms largest in European memory.
Journalist and cartoonist Joe Sacco, who illustrated a massive panorama of the first day of the battle, spoke to NPR in 2013. Here's what we wrote then:
"The battle began on 7:30 a.m., July 1, 1916, on the river Somme in France. There had already been a series of bombardments; British generals unloaded a week's worth of artillery, thinking it would decimate the Germans and allow British troops to move in easily.
"But while the bombardment was so loud that it could be heard in London, it hadn't been very effective; many of the shells were duds, and others just hadn't done the job. Then the barrage lifted, and the troops started to move.
" 'When all that noise quieted down, the Germans realized, OK, the shelling has stopped; let's get out of our dugouts and man our machine gun posts,' Sacco says. 'The British were marching towards them in a line, and the Germans just started firing on these troops.' "
It was a test of new battle tactics, and it was a devastating failure. More than 19,000 British soldiers were killed in the first day alone — the deadliest day in the history of the British military. Nearly 40,000 more were injured.
For some communities, the shock was particularly profound. Newfoundland, for instance — then a dominion of Great Britain — sent its own regiment to fight in the war, as historian Maureen Power told NPR's Weekend Edition a few days ago.
They went to the Somme, and nearly the entire regiment was wiped out in that single day.
"It stayed in our psyche after the war. We ended up basically going bankrupt and having to give up our own country," Power says. "We lost the best of the best in the young men that would have led Newfoundland into maintaining its own country, not having to join Canada [in 1949]."
In Britain, too, many towns saw the best of their best felled in one day. So-called pals battalions featured young men who knew one another — a plan concocted to increase the number of civilians volunteering for service.
When friends served together, they died together, too. The result was that some communities felt a disproportionate blow when the death notices started to arrive.
In honor of the anniversary of the Somme, British poet and nurse Molly Case has written a poem about the loss of pals battalions for the Royal British Legion.
The battle stretched on for months, as summer turned to fall.
Conditions were horrible. Difficulties with the supply chain meant soldiers were frequently hungry and thirsty. Rats terrorized the trenches.
Rain turned the battlefield into a nightmarish landscape of mud and corpses.
American-born poet Mary Borden, who ran a military hospital at the Battle of the Somme and earned medals for her bravery on the front lines, later wrote about that grim landscape, in "At the Somme: The Song of the Mud." One stanza reads:
"This is the hymn of mud-the obscene, the filthy, the putrid,
The vast liquid grave of our armies. It has drowned our men.
Its monstrous distended belly reeks with the undigested dead.
Our men have gone into it, sinking slowly, and struggling and slowly disappearing.
Our fine men, our brave, strong, young men;
Our glowing red, shouting, brawny men.
Slowly, inch by inch, they have gone down into it,
Into its darkness, its thickness, its silence.
Slowly, irresistibly, it drew them down, sucked them down,
And they were drowned in thick, bitter, heaving mud.
Now it hides them, Oh, so many of them!
Under its smooth glistening surface it is hiding them blandly.
There is not a trace of them.
There is no mark where they went down.
The mute enormous mouth of the mud has closed over them."
As summer turned to fall, the soldiers grew increasingly exhausted. The rain eventually turned to snow.
Britain introduced the tank for the first time, in September. It boosted morale, the BBC writes, but had little other impact on the course of the battle.
The BBC has a thorough timeline of events at the Somme — a grim litany of losses in a war of attrition. The battle didn't end until November.
By then, more than a million men had been wounded or killed.
And territorial gains?
The front moved less than 10 miles. By one calculation, the Allies paid 1,000 lives for every 100 yards they gained.
On Veterans Day a few years ago, NPR's Eleanor Beardsley visited the fields in France where the Battle of the Somme was fought.
"Today most of the battlefields have been plowed into fertile farmland. This time of year, French farmers are harvesting their sugar beets. In what is known as an 'iron harvest,' farmers continue to find grenades and other ordnance from World War I in their fields," she wrote.
"In a few areas the battlefields have been preserved. The once-deep trenches now appear like rows of bumpy, grass-covered stitches running across the land. Most of the towns, too, have been rebuilt, but visitors can buy picture postcards in the shops that show their utter destruction nearly a century ago."
It was a sobering trip, she wrote:
"A visit to a place like the Somme makes clear what all-encompassing warfare is about for those whose homes and farmland become the battlefields. World War I engulfed whole societies, ended a way of life and decimated an entire generation of young men.
"The landscape of the Somme is dotted with military cemeteries, and this is perhaps the most haunting part of the visit. The Franco-British monument to the missing at Thiepval is sad and beautiful. Its massive brick and marble structure rises incongruously out of the peaceful farmland and forest. Around the monument are rows of tombstones bearing the inscription, 'A Soldier Of The Great War — Known Unto God.'
"As I read the names of some of the 73,000 soldiers who were never found, inscribed on the sides of the monument, I was filled with such sadness at the futility of their loss.
"Only 21 years of peace separated the end of World War I from the beginning of World War II. These young men believed they were fighting valiantly for their nations in 'The War to End All Wars.' When, in fact, theirs was a prelude to an even deadlier conflict."
British Prime Minister David Cameron drew a line between World War I history and current affairs in Europe when he spoke to Parliament on Wednesday, The Associated Press reports.
"In many ways, there is a link between the current events we're discussing and what happened 100 years ago. It's the importance of keeping peace and security and stability on our continent," Cameron said. "We're going to be standing together and remembering the sacrifices all those years ago."
On Friday, at Waterloo Station in London, silent actors dressed as World War I-era troops passed out cards with the name, age and rank of the slain soldiers they represented, Reuters reports.
At London's Parliament Square, a gun salute was followed by two minutes of silence.
And at Thiepval in France, Cameron joined French President Francois Hollande and members of the royal family for a service of remembrance.
It wasn't just a time to think of Britain's dead, Cameron said, but of the lives lost on all sides.
"It is an opportunity to think about the impact of the devastation felt by communities across all of the nations involved, which left mothers without sons, wives without husbands and children without fathers," he said.