THIEPVAL, France ? Officers wept, families traced the steps of fallen relatives and church bells tolled across northern France on Saturday to mark the 90th anniversary of the Battle of the Somme, one of history’s worst bloodbaths.

Prince Charles called the World War I battle “a most profound shock” for Britain, and “an unutterable hell.”

Ceremonies around the world honored the soldiers of some 20 nationalities who died and those who survived the British-led push against German forces. Four months of vicious trench warfare ravaged the gentle hills of the Somme region and left more than 1.2 million on both sides dead, wounded or held prisoner.

The battle has nearly receded from living memory, but its legacy remains. Monuments – from simple markers to major museums – in the wheat fields and towns of the Somme serve as a reminder of how World War I changed Europe forever, and how young European unity is.

Britain feels the battle’s scars most deeply. July 1, 1916, was the deadliest day the British army ever saw, leaving 20,000 victims.