On 24 October, the Valjevo Offensive saw Potiorek launching a third invasion, this time reaching deep into northern Serbia, capturing Belgrade, the Serbian capital, on 2 December 1914.

Less than a year later, after combining the armies of Germany, Austria-Hungary and Bulgaria, the Central Powers returned for a massive offensive during the Serbian Campaign of 1915.

On 28 June 1914, Archduke Franz Ferdinand, heir presumptive to the Austro-Hungarian throne, and his wife Sophie were assassinated while visiting Sarajevo, the provincial capital of Bosnia-Herzegovina, formally annexed by Austria-Hungary.

On 25 July Franz Conrad von Hötzendorf, the Chief of the General Staff, gave the mobilisation order for the Austro-Hungarian units required for Case B, the war plan formulated against Serbia and Montenegro.

General Oskar Potiorek, the Balkanstreitkräfte commander leading the invasion of Serbia, began with a force of 460,000 soldiers spread across 19 divisions.

In opposition, Field Marshal Radomir Putnik commanded 400,000 Serbian troops, among whom were 185,000 seasoned veterans who had participated in the Balkan Wars of 1912–1913.

After facing fierce resistance from Serbian irregulars, the landing was aborted, and the monitors were redirected to the railway bridge connecting Serbia to the Habsburg Empire.

[13] At 5 am, Habsburg artillery in Bežanija and Semlin also began shelling the city and Kalemegdan using Krupp Howitzer and Skoda 305 mm mortars.

[10] On 24 August, the liberation of Šabac, the largest town in Mačva, marked the ultimate failure of the first Austro-Hungarian invasion of Serbia.

[17] Under pressure from its allies, Serbia conducted a limited offensive across the Sava River into the Austro-Hungarian region of Syrmia with its Serbian First Army.

"The Serbians, seasoned, war-hardened men, inspired by the fiercest patriotism, the result of generations of torment and struggle, awaited undaunted whatever fate might bestow."

A renewed Austro-Hungarian attack from the west, across the Drina river, began on 7 September, this time with both the Fifth Army in Mačva as well as the Sixth further south.

At that time, Marshal Putnik withdrew the First Army from Syrmia (against strong opposition), using it to deliver a fierce counterattack against the Sixth Army that initially went well but finally bogged down in a bloody four-day fight for a peak of the Jagodnja mountain called Mačkov Kamen, in which both sides suffered horrendous losses in successive frontal attacks and counterattacks.

Marshal Putnik ordered a retreat into the surrounding hills, and the front settled into a month and a half of trench warfare.

This was highly unfavourable to the Serbs, who had little in the way of an industrial base and were deficient in heavy artillery, ammunition stocks, shell production and footwear since the vast majority of infantry wore the traditional (though state-issued) opanaks[20] when the Austro-Hungarians had waterproof leather boots.

It was at that time that General Živojin Mišić was made commander of the battered First Army, replacing the wounded Petar Bojović.

On the Serbian side, a deadly typhus epidemic killed hundreds of thousands of Serb civilians during the winter.

After the Battle of Kolubara, the Serbian Parliament adopted the Niš Declaration (7 December 1914) on the war goals of Serbia: "Convinced that the entire Serbian nation is determined to persevere in the holy struggle for the defense of their homesteads and their freedom, the government of the Kingdom (of Serbia) considers that, in these fateful times, its main and only task is to ensure the successful completion of this great warfare which, at the moment when it started, also became a struggle for the liberation and unification of all our unliberated Serbian, Croatian and Slovenian brothers.