In the time of the Roman historian Tacitus (circa 55-117 A.D.), the area now watched over by NATO and Warsaw Pact armies was covered by a vast forest penetrated only by a network of narrow trails. A traveler could walk from Poland to the Rhine without even glimpsing the sun. As populations grew, forests gradually gave way to pastures and arable land. By the beginning of the twentieth century German geography—and military strategy—had undergone such radical transformation that Field Marshal von Schlieffen could see the North German Plain as a corridor for maneuvering thirty-four of the kaiser's divisions to outflank the French “if the last man on the right brushed the [English] Channel with his sleeve.”