On this day, 1940, the center of the city of Rotterdam was bombed by the Germans. They invaded the Netherlands a couple days earlier and the small Dutch army was resisting more than expected. To force a surrender, one of the first mass bombing raids of the war was executed. Later that day the Netherlands surrendered.
The medieval city center and a part of Kralingen was hit and a firestorm developed. In the end 900 people died and thousands were made homeless. Today is the official commemoration, but year round it is commemorated with these kinds of lights embedded in the sidewalk. They are a very clear marker of the border of the fires.
Though this is not the only reminder. Where other cities carefully rebuilt their historic buildings, Rotterdam started over. Canals were filled with the rubble of the buildings, existing streets were widened to make them car friendly, and all new buildings were built after the war. The result is a modern city center inside the brandgrens, and much older buildings outside of it. There is an absolute sense of crossing a border.