on 8 september, orders from okh reached guderian that xix army corps was to continue its operation more conservatively, to stay close by 3rd army's left flank and to not wander off too far to the east – incursions east of the line between ostrów mazowiecka and warsaw were outright forbidden. guderian and his superior, army group north's commander fedor von bock, protested against this restriction on the mobile forces. it turned out that gerd von rundstedt's army group south was not progressing nearly as swiftly as army group north had anticipated; the southern forces, far stronger than their northern counterpart, had failed to cross the vistula on any meaningful scale and had not yet conquered warsaw. the southern armored divisions had met rougher hardships than guderian's own units. 4th panzer division of xvi corps within 10th army had for example lost 57 out of 120 tanks, almost meeting a 50% casualty rate.