When British patrols probing German outposts found them unoccupied, the Allies began a cautious advance, slowed by German destruction of the transport infrastructure. The troubled transport situation behind the British front, which had been caused by mounting difficulties on the Nord railways, overloading and the thaw on roads made British supply problems worse. The Germans had the advantage of falling back over good roads to prepared defences, protected by rearguards. The German armies made an efficient withdrawal, although the destruction accompanying led a considerable amount of indiscipline. Defending villages as outposts, with most of the rearguard posted at the western exits, left them vulnerable to encirclement and attacks from commanding ground and the predictability of such methods, provided French and British troops with obvious objectives.