LONDON — With a dimming of the lights and ceremonies across this country and in Belgium, monarchs, princes, presidents and citizens commemorated on Monday the day 100 years ago when Britain entered World War I at the start of four years of carnage once called the war to end all wars.
Some took the moment to recall more modern crises in the Middle East and Europe that are rooted in the fighting between 1914 and 1918 that toppled empires and redrew the world map. Some dwelled on a vision of reconciliation among former foes.
“We were enemies more than once in the last century, and today we are friends and allies,” Prince William, the second in line to the British throne, told a ceremony in Belgium, referring to Germany and its allies in two world wars. At Westminster Abbey, prayers were said in English and German.
But today’s myriad wars haunted the commemorations, too.
“How can we remain neutral today when a people not far from Europe is fighting for their rights?” President François Hollande of France said in Belgium. “How can we remain neutral when a civilian airliner is brought down, when there is conflict in Iraq, Syria, Lebanon, Gaza?”
To echo those words, Prime Minister David Cameron urged Britons to extinguish the lights in their homes at 10 p.m. on Monday to leave a lone light or candle burning by 11 p.m. — the precise moment of the declaration of war on Germany.
In London, the lights went off at such landmarks as the Houses of Parliament and Tower Bridge over the River Thames. At Westminster Abbey, at a late-night ceremony attended by political leaders, a lone oil lamp at the tomb of the unknown soldier was extinguished at 11 p.m. by the Duchess of Cornwall, the former Camilla Parker-Bowles and wife of Prince Charles, the heir to the throne.
The fighting a century ago erupted after a series of interlocked events beginning with the assassination of Archduke Franz Ferdinand and his wife, Sophie, on June 28, 1914 — killings that set off a chain of events, driven by a complex web of alliances, that historians have described as Europe’s stumbling or sleepwalking into a cataclysmic conflict.
“Most were stumbling into the darkness, increasingly bound by the chains of their own and others’ making, their hope of avoiding war ever fading,” the Very Rev. Dr. John Hall, the dean of Westminster Abbey, said in a foreword to the order of service on Monday.
Many in Britain and elsewhere expected a quick end to the hostilities. But the war soon bogged down in trench warfare that consumed the energies and resources of nations at the cost of millions of lives.
Neutral at the beginning, the United States formally joined the war in 1917.
The writer H. G. Wells is often credited with coining the description of the conflict as “the war that will end war,” the title of an essay that became a jingoistic catchphrase, “the war to end all wars.”
As the conflict drew to a close, a more cynical view overtook that sentiment when David Lloyd George, the British prime minister at the time, is said to have remarked: “This war, like the next war, is a war to end war.”
The approach of the conflict in 1914 was commemorated on Sunday when Mr. Hollande and President Joachim Gauck of Germany embraced at a war memorial in the eastern French province of Alsace, near the frequently contested frontier between their nations. The occasion commemorated Germany’s declaration of war on France on Aug. 3, 1914, as German troops massed to invade neutral Belgium — the incursion that drew Britain into the war a day later.
No formal ceremonies were planned in Berlin, with German commemorations focused on Mr. Gauck’s attendance at ceremonies in lands once conquered by German soldiers. In Russia, President Vladimir V. Putin inaugurated a memorial in Moscow last week, and a museum is to open in St. Petersburg.
The scale of commemoration in Russia was unusual. Moscow usually focuses most of its commemorative efforts on World War II.