In the case of an organised withdrawal, Haig described a cautious follow up by advanced guards, in front of a main force moving periodically from defensive position to defensive position, always providing a firm base on which the advanced guards could retire. The conduct of the Anglo-French pursuit conformed to this model. General Franchet d'Espérey proposed an improvised offensive to Nivelle, who rejected the idea, in favour of strengthening the main French front on the Aisne. British heavy artillery had been moved north from the Fifth Army in January, ready for the offensive at Arras and had been partly replaced by inexperienced units from Britain. Divisions from the Fourth Army had been moved south, to take over former French positions and I Anzac Corps had been transferred to the Fifth Army to compensate for divisions sent north to the Third Army by 6 February, which left the Anglo-French forces in the area depleted.