The Serbian Campaign in the Great War started when the Austro-Hungarian Empire attacked the Kingdom of Serbia. The invasion became a disaster for the Austro-Hungarians when the Serbian army turned out to be a tougher opponent than anticipated. As such, the Serbians managed to succesfully defend their homeland in 1914. In the following year however the Austrians, with German help, managed to defeat the Serbians who retreated their armies to Greece, through Albania, where they could reorganize under the protection of the other Entente combatants. Serbia was under Central Powers occupation until the end of the war in 1918.
The approximately 200,000-strong Serbian Army was commanded by the elderly figure of Field Marshal Radomir Putnik. He would prove a formidable directing intelligence to Serbian military operations. The Serbian Army had recent experience in the Balkan Wars, but it was woefully ill-equipped for a full scale conflict with a major power.
Putnik, the son of a teacher, a gunner and former lecturer at the military academy, had stood at the apex of the Serbian army since 1903, as Chief of the General Staff for most of the time, and minister of war for the rest. He was 67 and suffered from emphysema, but he had two great strengths: he commanded the respect of the competing political groups in Serbia, and he was thoroughly familiar with the plans for his country's defense.
The Austrian army was strong on tradition, with many of its regiments claiming descent from those that had fought the Turks in the 17th century. With the steady expansion of the empire, however, it had been necessary to recruit increasingly from non-German elements. Problems of loyalty and language arose as German-speaking units became outnumbered by the other ethnic groups.
Despite the army’s shortcomings, Austria declared war on Serbia. A day later, Austrian warships on the Danube bombarded Belgrade as their 2nd, 4th and 6th armies under General Oskar Potiorek prepared to cross the rivers Sava and Drina. The Serbs, with 450,000 men supported by partly trained Montenegrins, all under command of Marshal Radomir Putnik, resolved to sell their lives dearly.
Potiorek had been passed over for the post of Chief of the General Staff in Conrad's favour. Thus, the most important difference between them was professional jealousy. Its symbol was the control and use of the 2nd army on Serbia's northern frontier: at an operational level its command was Potiorek's responsibility; at a strategic level its deployment was Conrad's. Not until his formal appointment did Potiorek know he was not to have full use of the 2nd army. It was a decision he never accepted.
Under orders from Conrad, the Austrian Fifth Army crossed the Drina River border. The Austrians were determined to finish the matter quickly and saw no problem in defeating their weak opponents. In doing so they not only attacked with only half their available strength, but worse still they were advancing into the rough country of western Serbia rather than the northern plains. Putnik was initially taken aback by the attack, imagining it to be a feint, but recovered swiftly to rush in reinforcements. The resulting four-day battle forced the Austrians to fall back.
Small-scale Serbian offensives incurring onto Austro-Hungarian territory were markedly less successful, so the next major act occurred when the Austrians launched a twin-pronged attack by the Fifth and Sixth Armies across the Drina to secure a firm bridgehead. The fighting was murderous, as Putnik marshalled his smaller forces as best he could, launching counterattacks to disrupt the Austrian advance. The fighting climaxed in another four-day battle in the mountains, marked by a series of sanguinary frontal assaults by both sides. Eventually numbers told and the Serbs fell back, and trench warfare took its iron grip.
The Austrians attacked again, when they used their artillery superiority to push the Serbs back. The Serbs withdrew slowly at first but pulled right back to shorten their front. This meant that the capital, Belgrade, had to be abandoned, which the Austrians duly entered. Putnik launched a counterattack crashing first into the Austrian Sixth Army. The results were spectacular: the Sixth Army broke and fell back in complete disorder. At this point Putnik turned his forces to the Fifth Army, which also crumbled under the pressure. Facing complete defeat, the Austrian armies retreated back to the borders. Belgrade was recaptured by the Serbs.
The campaign had been a fiasco for the Austrians: nothing had been achieved, thousands of men had died and the Serbs still preened themselves across the border, acting as beacons for all the disaffected minorities within their domains. Austria-Hungary had the war with Serbia she had craved in July 1914: distracted by the menace of the Russian armies, she did not have the strength to win it. After the Austrian defeat, the Serbian front was quiet for almost a year.
Before 1915 ended, there was one enemy that Erich von Falkenhayn, the German Chief of Staff, accepted could be dealt with permanently in the period of grace granted by the string of defeats suffered by Russia in 1915: Serbia. He allocated the German Eleventh Army to the task of joining with Conrad’s Austrian armies in an assault on Serbia under the overall command of August von Mackensen. They were also to be helped by a new ally: the Bulgarians, who had been greatly impressed by the triumph at the Battle of Gorlice-Tarnów and had begun serious negotiations with a view to joining the war on the side of the Central Powers.
The odds overwhelmingly disfavored the Serbs. Serbia's only hope of altering the balance lay in attracting Entente troops into the Balkans, via the Greek port of Salonika. In the hope that an Entente intervention might now allow them to defeat the Bulgarians in the south before the Germans and Austrians developed their attack in the north, the Serbs made a plea to the French and British to endorse the Salonika plan. The plan was put into action, with the Greek Prime Minister Eleftherios Venizelos’ support. It was now too late, however, to save Serbia. Nonetheless the Macedonian front of World War 1 had opened, with the Entente and Central Powers fighting for supremacy in the Balkans.
Mackensen launched his assault with a heavy bombardment pounding the Serbian positions all along the Sava and Danube Rivers. The German Eleventh Army and Austrian Third Army managed to establish significant bridgeheads and, once the integrity of the Serbian river lines was broken, their weight of numbers soon hurled the Serbians back and Belgrade fell again. Then, the Bulgarian First and Second Armies invaded Serbia from the east. The campaign that ensued was brutal.
The Central Powers had underestimated both the sheer bloody-mindedness of the Serbs and the impact of the winter weather in the mountainous areas of the central Balkans. With their leader, Field Marshal Radomir Putnik, travelling in a sedan car, the Serbians eluded attempts to cut them off and fell back through the mountain passes into Montenegro and Albania. They then managed to reach the Adriatic where they were evacuated by the British to Greece. The Central Powers had captured Serbia, but the Serbian Army would fight on regardless.
After the Bulgarians were defeated during the Vardar Offensive of September 1918, the Bulgarian Army collapsed, and with them came the collapse of the Central Powers in the Balkans. During October 1918 Serbian forces advanced toward Serbia liberating the country as the defeated Bulgarians and Austro-Hungarians retreated.