Early in September they prepared a second army of invasion. Before I follow its fortunes, I want you to look closely at the little map on the opposite page. You notice that from the railway uniting the Lower Drina with Shabatz on the Save, right away to the southern frontier, the country is criss-crossed in all directions by great uplands, almost as difficult for an army to traverse as those of Georgia. The river valleys alone give access to the interior, and afford railway routes. There are a few good government highways, but most of the roads are mere tracks, which in wet weather become quagmires. When once the autumn rains set in, the work of transport in Serbia is greatly hampered.
All the lowland parts of Serbia lie along the right bank of the Save and the Danube. This district is open, though hilly. East of Shabatz the lowlands form a rough triangle, with the course of the river Save as its base, and the highland town of Valjevo, on the river Kolubara, as its apex. It was across this triangle of easy country that the Austrians made their second invasion.
The Serbians did not wait for the Austrian attack, but pushed over the plain, and in the darkness of night on 6th September crossed the Save at several points, and made a dash on the Hungarian town of Semlin, opposite Belgrade. They occupied the place on the 10th, but were unable to hold it, and were driven back with heavy losses into the Tser Mountains, which you see marked on the map.
The Austrian commander-in-chief proposed to turn the Serbian left, and at the same time hurry forward mountain troops to Valjevo, and thus cut off the retreat of the Serbian army. During September and October attacks and counter-attacks were constant, but neither side made much headway. The Austrians could not drive the Serbians off the crests of the mountains, and the Serbians could not drive back the Austrians, who were advancing up the Drina to turn the Serbian left. At length, however, the Serbian ammunition began to fail, and the Austrian numbers began to tell. On 6th November the enemy won the summits of the Tser Mountains, and the Serbians, in order to avoid being enveloped, were forced to retreat eastwards. This they did in perfect order.