Complex sentence: He saw extensive action during World War II, combating the U-Boat threat during the Battle of the Atlantic, where his most notable achievement was the capture of the German submarine "U- 505" on June 4,1944.
Split into simpler sentences: He saw extensive action during World War II, combating the U-Boat threat during the Battle of the Atlantic. His most notable achievement was the capture of the German submarine "U- 505" on June 4,1944.