Leaders and dignitaries from former World War I enemies are uniting to mark the centenary of the first big battle on the infamous Flanders Fields which helped set the stage for four years of bloodletting by hundreds of thousands on the Western Front.

German Chancellor Angela Merkel spoke Tuesday of the "immeasurable sacrifice" the war caused and said all sides "cannot but be grateful for how much has changed since."

World War I claimed an estimated 14 million lives, including 5 million civilians and 9 million soldiers. At least 7 million troops were left permanently disabled.

The combatants dug in along a line of fortified trenches that extended from the North Sea to Switzerland — a front line that remained essentially unchanged throughout the war.