Against fierce resistance, Rokossovsky attacked across the Narew on 14 January; on 20 January, he received orders to swing the axis of his advance northward toward Elbing. This sudden change of direction caught Reinhardt and Hossbach by surprise; on Rokossovsky's right flank, the 3rd Guards Cavalry Corps captured the major town of Allenstein on 22 January, threatening the rear of Hossbach's formation. On 24 January, Rokossovsky's leading tank units had reached the shore of the Vistula Lagoon, severing land communications with the rest of German armed forces for the entire 4th Army along with several divisions of the 2nd Army which were now trapped in a pocket centered on East Prussia. On the same day, Hossbach began to pull his units back from the fortified town of Lötzen—a center of the East Prussian defence system—and through a series of forced marches attempted to break out westward.