with the threat of a polish counter-offensive along the river bzura, fedor von bock convinced okh to allow xix army corps to operate at its own pace on 3rd army's left flank. once more, von bock instructed guderian to set his sights onto brest-litvosk, far east of the line between ostrów and warsaw previously given by okh and deep in the polish rear. miscommunications with the infantry-based xxi army corps during their narew crossing caused casualties; german infantry units, if unsupported by armored or air forces, had great trouble overcoming well-entrenched polish defenders.