Speaking at the Canadian War Museum in Ottawa, Harper spoke of how the Great War decimated a generation of young Canadians.
"Yet amid the appalling loss, by any measure, Canada as a truly independent country was forged in the fires of the First World War.
"That is to say, when the great nations of the world gathered, we must never forget that our place at the table was not given to us — it was bought and paid for," Harper said, referring to a number of battles, including Passchendaele and Vimy Ridge, as well as Ypres, where John McCrae wrote In Flanders Fields.
Earlier Monday, Harper placed a wreath at the tomb of the unknown soldier at the National War Memorial.
“It is a time to remember and honour the sacrifices and tremendous achievements of the more than 650,000 brave Canadians and Newfoundlanders who left their families and the comfort of their homes to serve their King and country, as well as to preserve the universal values of freedom, peace and democracy that we hold most dear," Harper said in an earlier statement.
“The dedication, courage and determination demonstrated by our brave soldiers, sailors and airmen, who stood shoulder-to-shoulder with like-minded allies to fight for what they believed in, resulted in Canada emerging as a proud, victorious nation with newfound standing in the world."
Gov. Gen. David Johnston said in a statement that this important centenary also allows Canadians an opportunity to honour the sacrifices of the estimated 425,000 Canadians who served overseas between 1914 and 1918.
"Their service contributed to the outcome of the war and to Canada's emergence as an independent nation," Johnston said.
"As Governor General and commander in chief of Canada, and as a father and grandfather, I encourage all Canadians to reflect upon the dedication of those who served in the First World War and to remember their sacrifices," he said.
The declaration of war came in a telegram from the British government to the then Governor General. By the time it was over, more than 60,000 Canadians were killed, and many thousands more returned home broken.
Jonathan Vance, a professor at Western University in London, said Canadians had an expectation in 1914 that they would be joining the battles.
Many, he said, even celebrated the fact, largely because they believed the war would be short-lived.
Belgium, France, Britain, Germany united
Meanwhile, former enemies united Monday to commemorate the 100th anniversary, with Belgium, France, Britain and Germany standing together in a spirit of reconciliation.
Belgian King Philippe and Queen Mathilde welcomed German President Joachim Gauck under cloudy skies for the late-morning ceremony at the Cointe allied memorial amid pomp and military honour. The former enemies sat united, listening and applauding each other's speeches.
Germany invaded neutral Belgium on Aug. 4, 1914, as part of a planned attack on France. By nightfall, Britain had joined the war.
"It opened Pandora's Box," said Gauck, who acknowledged that it "is anything but self-evident to stand and talk to you on this day."
The war wasn't expected to last long. But instead of weeks, the continent was plunged into hardship and misery for more than four years.
Gauck will join British Princes William and Harry at the Saint Symphorien cemetery late Monday for a similar remembrance. In Britain, there was a ceremony in Glasgow, Scotland, and a late-evening candlelight vigil at London's Westminster Abbey.
The Great War claimed an estimated 14 million lives, including five million civilians as well as nine million soldiers, sailors and airmen from 28 countries. At least seven million troops were left permanently disabled.
On Sunday, an intense hug between Gauck and French President Francois Hollande during a remembrance ceremony in eastern France close to the German border sealed again the friendship between the two neighbours, who have become the cornerstones of the European Union.
Monday's ceremony in Liege was significant since the battle for the forts around the city meant the first delay for Germany's enveloping move through Belgium, the so-called Schlieffen Plan strategy to defeat France in a matter of weeks.
Liege held much longer than expected, and allowed the allied forces to gather strength and keep Germany at bay within dozens of kilometers of Paris.
Gauck called the German plan "hapless" and deplored German actions against civilians and cities its forces passed through during the early weeks of the war.
By the end of autumn 1914, both sides dug in, and from the early battles, the war quickly changed into trench warfare on the Western Front, with hundreds of thousands of casualties in a barren landscape where poison gas often wafted through the air.
The U.S. joined the allies against the German and Austro-Hungarian empires in 1917 and provided a decisive impetus to break the deadlock before the Nov. 11, 1918, armistice.