Complex sentence: After evacuation from France in 1940 having been part of the'Second BEF' that remained in France after Dunkirk, eventually being evacuated in late June 1940 during Operation Ariel, it trained as a mountain warfare formation but was never used in that role.
Split into simpler sentences: After evacuation from France in 1940 having been part of the'Second BEF' that remained in France after Dunkirk, eventually being evacuated in late June 1940 during Operation Ariel. Back in the UK, it trained as a mountain warfare formation but was never used in that role.