on 12 september, xix army corps was for the first time itself threatened by enemy encirclement. the remains of the battered polish narew operational groups, supported by the podlaska cavalry brigade, were now in xix army corps' left rear, in the grodno and białystok areas. guderian made a crucial tactical error when he moved the corps hq across the river narew too prematurely on 10 september, which made it difficult for the rest of his staff to fulfill their tasks. several of the roads in the area had not yet been secured by german infantry units that served as the armor's rear guard and were thus still in polish hands. guderian himself had to be evacuated out of the area by german motorcyclists to avoid capture by the polish forces.