These photos are said to be from early 1940 in an unknown location along the New Dutch Waterline. The Waterline was a defensive line designed to protect the Netherlands from land-based attacks. It is named the Waterline because dykes, flooded land and moats were used to keep attackers at bay. The original Old Hollandic Waterline dated back to the 17th century. It was rebuilt in the 19th century after the Napoleonic wars and again modernized after WWI.
During winter it makes sense that soldiers would skate over the ice to quickly move about the frozen lakes or to pursue a retreating enemy.