During World War II cities protected themselves from nightime air raids with blackouts. All streetlights were turned off and cars had to drive with their headlights off. Citizens were banned from using external lights and had to carefully cover all windows and openings with curtains or covers to ensure that no light shone outside. This made it much more difficult for the bombers to locate and hit their targets. All major German towns and cities except one operated this policy.
Konstanz in Baden-Württemberg lies close to the Swiss border. Switzerland was neutral and allied bombers were ordered to avoid it. Its towns and cities were well lit. By deliberately keeping all its lights on Konstantz was able to deceive the attackers into thinking it was in Switzerland so they flew on past.