World leaders commemorated the 100th anniversary of the outbreak of World War I by warning of lessons to be learned in the face of today's many crises, including Ukraine.
"Peace has to be a shared goal," Belgium's King Philippe told leaders gathered in his country's eastern city of Liege on Monday.
"World War I reminds us to reflect on our responsibility... to bring people together."
Leaders from across Europe attended the commemoration at the Allied War Memorial of Cointe, a tower complex overlooking the city alongside a weathered grey-stone church painted with white doves for the occasion.
French President Francois Hollande recalled Germany's invasion of neutral Belgium in early August 1914 that turned what had been a localised Balkans war into a global conflagration, raising current day parallels.
"How can we remain neutral today when a people not far from Europe is fighting for their rights?" Hollande said, clearly referring to the Ukraine crisis.
"How can we remain neutral when a civilian airliner is brought down... when there is conflict in Iraq, Syria, Lebanon, Gaza?"
"We cannot remain neutral... Europe must live up to its responsibilities with the United Nations," he told an audience that included Britain's Prince William and wife Catherine, heads of state and representatives of some 80 countries.
The leaders gathered in the industrial town of Liege because dogged fighting there had barred the way to invading German troops in the early days of August 1914.
Liege's fierce resistance derailed Berlin's plans for a quick victory, while Germany's invasion of Belgium formally brought Britain into the war, as interlocking alliances that were meant to preserve the peace plunged Europe into the abyss.
Early Monday, Australian Prime Minister Tony Abbott and his New Zealand counterpart John Key paid tribute to the tens of thousands of their countries' soldiers who died far from home.
The conflict "was the most cataclysmic event in human history," Abbott said, "and arguably gave rise to communism, to Nazism, to World War II and the Cold War".
Meanwhile, people were being asked to turn off their lights from 2100 GMT to 2200 GMT, inspired by British foreign secretary Edward Grey's famous remark on the eve of war.
"The lamps are going out all over Europe; we shall not see them lit again in our life-time," he said in despair.
About 17 million soldiers and civilians lost their lives in the war.