Complex sentence: The historians wrote that the Franco-British armies conducted attacks from 17 December between Arras and Armentières but that by 20 December the attacks had been contained but skirmishing continued around Carency, Ecurie, Neuve Chapelle and La Bassée.
Split into simpler sentences: The historians wrote that the Franco-British armies conducted attacks from 17 December between Arras and Armentières. By 20 December Allied attacks had been contained but skirmishing continued around Carency, Ecurie, Neuve Chapelle and La Bassée.