Across Europe, families and veterans came together today to somberly commemorate the start of World War I on Aug. 4, 1914. Many of the ceremonies involve poppies, which have become a symbolic remembrance of fallen soldiers.

Poppies became a powerful metaphor in the wake of the publication of the poem “In Flanders Fields,” by Colonel John McCrae, a Canadian surgeon. The trenches of Flanders, located between Belgium and northern France and known as the Second Battle of Ypres, saw heavy fighting. McCrae was on the front lines and saw one of his friends die during the battle. According to Canada’s Veterans Affairs Office, wild poppies were growing around the makeshift graves of soldiers and provided the inspiration for McCrae’s poem. “In Flanders Field” was published in Punch magazine in December 1915.

The remembrance poppy is part of the tradition commemorating fallen troops in Canada, the U.S. and the U.K. The “Buddy Poppy” is the official flower of the Veterans of Foreign Wars.

As part of the WWI centenary, artist Paul Cummins was commissioned to fill the moat of the Tower of London with ceramic poppies, titled "Blood Swept Lands and Seas of Red." Throughout the summer, the moat will be filled with more poppies, eventually totaling 888,246 ceramic poppies, to symbolize the number of British casualties in the conflict.

On Monday, a ceremony was held for the WWI 100th anniversary at Liege, Belgium. The country's King Philippe and Queen Mathilde, French President Francois Hollande, German President Joachim Gauck and Britain's Prince William and Catherine, the Duchess of Cambridge, were among guests at the ceremony. Prince Charles and British Prime Minister David Cameron attended an event in Scotland while Prince Harry was at an event in England. In Britain, they will recognize the WWI centenary with an hour of darkness as they will shut off the lights beginning at 5 p.m. EDT., reports Reuters. Many remaining signs of WWI can still be seen across Europe, from Italy's legacy to buried, unexploded shells in France.

McCrae's "In Flanders Field" can be read below.

In Flanders fields the poppies blow
Between the crosses, row on row,
That mark our place; and in the sky
The larks, still bravely singing, fly.
Scarce heard amid the guns below.

We are the dead. Short days ago
We lived, felt dawn, saw sunset glow,
Loved, and were loved, and now we lie
In Flanders fields.

Take up our quarrel with the foe:
To you from failing hands we throw
The torch; be yours to hold it high.
If ye break faith with us who die
We shall not sleep, though poppies grow
In Flanders fields.