Complex sentence: One of which was the Action of 21 May 1918 when she was credited with sinking a German submarine off Spain but in she reality only damaged the U-boat which was interned by Spain a few days later.
Split into simpler sentences: One of which was the Action of 21 May 1918 when she was credited with sinking a German submarine off Spain. However, later it was found that the U-boat was only damaged but had to be interned by Spain a few days later.