World War I: Combined Austro-Hungarian and German Central Powers, reinforced by the recently joined Bulgaria launched a new offensive against Serbia under command of August von Mackensen .

World War I or the First World War, also known as the Great War, was a global conflict between two coalitions: the Allies and the Central Powers. Main areas of conflict included Europe and the Middle East, as well as parts of Africa and the Asia-Pacific. There were important developments in weaponry including tanks, aircraft, artillery, machine guns, and chemical weapons. One of the deadliest conflicts in history, it resulted in an estimated 30 million military casualties, plus another 8 million civilian deaths from war-related causes and genocide. The movement of large numbers of people was a major factor in the deadly Spanish flu pandemic.

The Central Powers, also known as the Central Empires, were one of the two main coalitions that fought in World War I (1914–1918). It consisted of the German Empire, Austria-Hungary, the Ottoman Empire, and the Kingdom of Bulgaria; this was also known as the Quadruple Alliance.

The Tsardom of Bulgaria, also known as the Third Bulgarian Tsardom, usually known in English as the Kingdom of Bulgaria, or simply Bulgaria, was a constitutional monarchy in Southeastern Europe, which was established on 5 October [O.S. 22 September] 1908, when the Bulgarian state was raised from a principality to a tsardom.

The Kingdom of Serbia was a country located in the Balkans which was created when the ruler of the Principality of Serbia, Milan I, was proclaimed king in 1882. Since 1817, the Principality was ruled by the Obrenović dynasty. The Principality, under the suzerainty of the Ottoman Empire, de facto achieved full independence when the very last Ottoman troops left Belgrade in 1867. The Congress of Berlin in 1878 recognized the formal independence of the Principality of Serbia, and in its composition Nišava, Pirot, Toplica and Vranje districts entered the South part of Serbia.