Complex sentence: He advised adopting a defensive military policy in response to superior British lines of communications which would ensure continued numerical superiority in any contested theatre making it impossible for the Yildirim Army Group to go on the offensive.
Split into simpler sentences: He advised adopting a defensive military policy, in response to superior British lines of communications, which would ensure continued numerical superiority in any contested theatre. This imbalance would make it impossible for the Yildirim Army Group to go on the offensive.