Complex sentence: Though planned as a simple tactical withdrawal and executed in good order, the British retreat from Mons lasted for two weeks, travelled and took the BEF to the outskirts of Paris, before it counter-attacked in concert with the French, at the Battle of the Marne.
Split into simpler sentences: Though planned as a simple tactical withdrawal and executed in good order, the British retreat from Mons lasted for two weeks, and covered. The BEF retreated to the outskirts of Paris, before it counter-attacked in concert with the French, in the Battle of the Marne.