Complex sentence: The second German withdrawal on 11 March took place during the British bombardment preparatory to an attack and was not noticed until the night of 12 March, when British patrols found the line empty between Bapaume and Achiet le Petit and strongly held on either flank.
Split into simpler sentences: The second German withdrawal took place on 11 March, during a preparatory British bombardment and the British did not notice until the night of 12 March. Patrols found the line empty between Bapaume and Achiet le Petit and strongly held on either flank.