To cut it was to deprive those armies of reinforcements, munitions, and other supplies coming from the south. Furthermore, possession of the Morava-Maritza trench would never be secure so long as Serbia and her allies held the Yardar depression, for at any moment they might launch a bolt along this natural groove which would sever the Orient Railway at Nish and thus undo all that had been accomplished through the new alliance with Bulgaria. For the Teuton-Bulgar forces the capture of the combined Morava and Yardar valleys was a single military problem. Let us examine the physiographic features which serve as natural defenses of this important trench.
The Northern Defenses. The Morava valley is widely open to the north and is there bounded on both sides by comparatively low hills. An enemy securing a foothold in the rolling country to the east or west could enter from either of these directions as well as from the north, just as the Orient Railway coming from Belgrade enters the valley from the west, twenty-five miles above its mouth. Hence an effective barrier against attack from the north must cover more than the actual breadth of the northern entrance to the valley. Such a barrier is provided by the natural moat of the Save and Danube Rivers which protects the entire northern frontier of Serbia; and by the hills south of the moat which, as one progresses southward, rise into a wild, mountainous highland.
South of Mitrovitza
The Save is a late-mature river swinging in great meanders across a broad, marshy flood-plain. The extensive swamp-lands on either side of the river are difficult to traverse at any time, while the flood waters which spread over the lowland in spring and autumn often make the barrier quite impassable except at Mitrovitza (not to be confused with the Mitrovitza near the Kosovo Polye referred to farther on). South of Mitrovitza and west of Shabatz the marshy peninsula between the Drina and the Save is called the Matchva and is famous for its inhospitable character. In volume the Save is of sufficient size to constitute an obstacle against invasion, but for purposes of navigation it suffers from its overlong meandering course and from frequent shifting of channels and sand-bars. At no point is the stream fordable, and at Belgrade alone is it crossed by a bridge.