Complex sentence: In 1937, during the Spanish Civil War, aircraft from the Francoist expeditionary forces bombed Vallcarca, as it was an important military target due to the cement plant and its harbor which connected the town with Mallorca.
Split into simpler sentences: In 1937, during the Spanish Civil War, aircraft belonging to the Francoist expeditionary air forces bombed Vallcarca. Back then it was an important military target owing to the cement plant and its harbor which connected the town with Mallorca.