On the 16th the entire line of formidable frontier defenses centered on Kenali had to be abandoned by the German and Bulgarian troops, in which operation they lost heavily. They then retreated across the Viro River, west of the railroad and across the Bistritza River to the east of the line, so that the Russians, following them up closely, succeeded in arriving within four miles of the city. Meanwhile the Serbians, in the mountains east of the swamps which protected the plain along the Cerna, were rushing rapidly on in their effort to swing around to the northeast of the city before the enemy should be able to intrench himself among the rolling hills that bound the northern extent of the plain. It was significant that among the prisoners were a number of members of regiments which had been fighting, only a week previously, upon the Dobrudja front under Mackensen.
A glance at the map will show that Monastir was now practically in the hands of the Allies, that it would be impossible for the Germans and Bulgarians to hold it any longer. As has already been explained, the plain or valley near whose head it stands is a broad, level region which here penetrates the mountainous interior of this portion of the Balkan Peninsula. To the eastward it is bounded by low, rolling foothills, which gradually rise into high, rocky mountains or heights. On the west there are no foothills: the mountains rise abruptly, stretching south almost to Kastoria. The railroad, after leaving Banitza, in the foothills, swings around into the plain, but under the shadows of the high ridges on the western side. Up toward the head of the plain these mountains curve slightly inward. About ten or fifteen miles below the point where they meet the rolling foothills, thus forming the head of the valley, the city of Monastir lies, some of its outlying suburbs being plastered up against the base of the mountains.
An army occupying the heights above would absolutely dominate the city; its artillery could pound it to a wreck within a few hours.
It was along these heights on the western edge of the plain that the French and the Serbians had advanced, driving the Bulgarians and Germans before them. Just at Monastir these heights are especially high and jagged, and the Bulgarians and Germans might very well have held out here against the enemy for a much longer period. But the foothills over on the eastern side of the plain had been passing into the hands of the Serbians operating in that region. These forces were now passing to the northward of the city, though the marshy plain still intervened. They were advancing toward the head of the valley. Should they succeed in reaching this point, where the highway to Prilep passed, they would cut off the retreat of the Bulgarians.