Complex sentence: But after only a few weeks, the dugout was lost when the Germans undertook the Battle of the Lys in April 1918, but was regained in September 1918, when its last occupants became the 2nd Battalion of the Worcestershire Regiment.
Split into simpler sentences: But after only a few weeks, the dugout was lost when the Germans undertook the Battle of the Lys in April 1918. It was recaptured in September 1918, when its last occupants became the 2nd Battalion of the Worcestershire Regiment.