Complex sentence: During the Battle of Dunkirk and Siege of Calais (1940), Richthofen supported the advance of Army Groups A and B. However, they were frequently meeting Royal Air Force (RAF) fighters, flying across the Channel.
Split into simpler sentences: During the Battle of Dunkirk and Siege of Calais (1940), Richthofen supported the advance of Army Groups A and B in these operations. His command were frequently meeting Royal Air Force (RAF) fighters, flying across the Channel.