Guernica is the site of a particularly horrifying chapter in Spain’s history. Here´s the Cliff Notes version of the story. Before (and after) WWII Franco was the dictator of Spain. He was friends with Hitler. Franco had problems with insurgents both in the Basque country and in Catalonia. He gave Hitler permission to test his military technology and techniques, in both locations, but particularly in the Basque Country. They chose a village called Guernica. On a market day, April, 1937, Hitler´s bombers strafed the town 5 times, taking out the bridges so no one could escape and then killed hundreds of citizens and destroyed most of the buildings and churches. More details are here.
The Spanish government regretted this action later, and granted the Basque region the ability to collect taxes, with a portion being passed on to the Spanish Government.