Prelude From 29 July to 11 August, the Austro-Hungarian army launched a series of artillery attacks in northern and northwestern Serbia and subsequently managed to exploit the bombardments by constructing a system of
pontoon bridges across the Sava and
Drina rivers. The Serbians knew that it was impossible for their forces to line the Austro–Serbian border, which extended . Putnik thereby re-ordered the Serbian army to fall back on a traditional line of defence as he grouped the bulk of his forces in
Šumadija, from where they could rapidly move either north or west. Strong detachments were posted in the towns of
Valjevo and
Užice, and outposts were stationed at every important point on the frontier. At this stage, all the Serbian General Staff could do was wait until the Austro-Hungarian invasion plan materialized. Belgrade,
Smederevo and
Veliko Gradište continued to be subjected to more vigorous artillery bombardments, and several attempts to cross the Danube resulted in heavy Austro-Hungarian losses. The bulk of the Austro-Hungarian forces were stationed in Bosnia, and the Serbian General Staff were not misled by these feints on the Danube. Subsequently, the Austro-Hungarians attempted to cross the Drina at
Ljubovija and the Sava at
Šabac, and these attacks were seen as more significant. On 12 August, Austro-Hungarian troops entered Serbia through the town of
Loznica, crossing the Drina. There, and in the village of
Lešnica, the Austro-Hungarian
13th Army Corps made a crossing, while on the same day the Austro-Hungarian
4th Army Corps crossed the Sava to the north of Šabac, while other Austro-Hungarian troops crossed the Drina. The town of Šabac was quickly taken. By 14 August, over a front of about , the Austro-Hungarians had crossed the rivers and converged on Valjevo. The Austro-Hungarian 2nd and
5th Armies moved towards Belgrade, where they encountered the Serbian 1st, 2nd and 3rd Armies. On 15 August, Putnik ordered his forces to counterattack.
Combat Around 23:00 on 15 August, elements of the Serbian
1st Combined Division encountered outposts set up by the invading Austro-Hungarian army on the slopes of
Cer Mountain and fighting erupted. The Austro-Hungarian positions were lightly held, and their defenders were driven back away from the mountain. By midnight, fierce clashes between the Austro-Hungarians and the Serbs were underway and chaos ensued in the darkness. By the morning of 16 August, the Serbians had seized the Divača Range and dislodged the Austro-Hungarians from their positions in the village of Borino Selo. The Austro-Hungarians, who had suffered heavy casualties during the fighting, retreated in some disorder. As the day progressed, the Serbs drove the
21st Infantry Division off the slopes of Cer to prevent it from linking with the 2nd Army in Šabac. On 17 August, the Serbs attempted to retake Šabac, but their efforts failed. The 1st Combined Division attacked the villages of Trojan and Parlog before moving on toward the small town of Kosanin Grad. Elsewhere, the Austro-Hungarians succeeded in repulsing the Serbian 3rd Army, forcing it to manoeuvre one of its divisions to protect the approach to the town of Valjevo, which was threatened by the
42nd Home Guard Infantry Division. In the early morning of 18 August, the Austro-Hungarians launched another attack, with the intention of pushing the
1st Šumadija Division off the Šabac bridgehead to allow the 5th Army to advance. However, the attack failed as the Serbs defeated the Austro-Hungarians at the Dobrava River, forcing their surviving soldiers to withdraw. Elsewhere, the Serbian 2nd Army's counter-offensive continued along the Cer and Iverak, with the 1st Combined Division attacking the village of Rašulijača and coming under severe pressure at Kosanin Grad. The first Serbian assault was fought off, but a wave of further attacks followed throughout the night. In the early morning of 19 August, the Serbs finally defeated the Austro-Hungarians and seized the small town. The
1st Morava Division drove the
9th Infantry Division from its position and fought off the division's subsequent counterattack, inflicting heavy losses. The 4th Corps renewed its attack against the Šumadija Division, forcing the Serbs to withdraw having only sustained light casualties. Because the 4th Corps did not break the Serbs, the Austro-Hungarian division was unable to alter the direction of its advance towards Cer Mountain, since doing so would have put the Šumadija Division in a position to attack the 4th Corps from the rear. As a result, the 4th Corps was unable to join other Austro-Hungarian forces fighting at Cer. The Serbs retook Rašulijača at noon, and the 1st Combined Division exploited this to advance towards Lešnica. Meanwhile, the 1st Morava Division attacked Iverak and managed to drive the Austro-Hungarians back. The village of Velika Glava fell to the Serbs before midday, and by late afternoon, the Rajin Grob ridge had been retaken. At around this time, the Austro-Hungarians began retreating with increasing rapidity, their will and cohesion apparently shattered. The 3rd Army had similar success, routing the
36th Infantry Division and forcing it to retreat in considerable disorder. The Serbs then moved to pursue the fleeing Austro-Hungarians all along the front. By 20 August, Austro-Hungarian forces were fleeing across the Drina River, still being pursued by the Serbs back into Bosnia, with the entire 5th Army being forced across the Austro-Hungarian side of the river. Many Austro-Hungarian soldiers drowned in the water as they fled in panic. Serbian military reports announced that "the enemy is withdrawing in the greatest disorder." Putnik then notified King
Peter in a telegram, saying "The main enemy has been defeated in Jadar and on Mount Cer, and our troops are in hot pursuit." Upon their triumph at Cer Mountain, the Serbs sought to recapture the heavily fortified town of Šabac. Violent clashes occurred on 21 and 22 August, during which Serb forces fought their way to the western approaches of the town. By 23 August, the Serbs had encircled the town and that evening they brought up their siege artillery. On 24 August, Serbian forces entered Šabac and discovered that the Austro-Hungarians had decamped the previous night. By 16:00, the Serbs reached the banks of the Sava River, bringing the first Austro-Hungarian invasion of Serbia to an end. ==Casualties==