Dutch soldiers on ice-skates, 1940
The Waterline was a defensive line made up of dykes, flooded land and moats designed to protect the Netherlands from western attacks. The original Old Hollandic Waterline dated back to the 17th Century. It was rebuilt in the 19th Century after the Napoleonic wars and again modernised after WWI. During Winter soldiers would skate over the ice to pursue a retreating enemy.
Credit to WeaponsCache @ Retronaut.