what happened during the wwi in this city?
Most of the Balkan offensives occurred near Belgrade - Austro-Hungarian monitors shelled Belgrade on 29 July 1914.
Belgrade
Most of the subsequent Balkan offensives occurred near Belgrade. Austro-Hungarian monitors shelled Belgrade on 29 July 1914, and it was taken by the Austro-Hungarian Army under General Oskar Potiorek on 30 November. 
where is belgrade located? At the confluence of the Sava and Danube rivers and the crossroads of the Pannonian Plain and the Balkan Peninsula.
wiki:5351155
Belgrade [SEP] World War I
The First World War began on 28 July 1914 when Austria-Hungary declared war on Serbia. Most of the subsequent Balkan offensives occurred near Belgrade. Austro-Hungarian monitors shelled Belgrade on 29 July 1914, and it was taken by the Austro-Hungarian Army under General Oskar Potiorek on 30 November. On 15 December, it was re-taken by Serbian troops under Marshal Radomir Putnik. After a prolonged battle which destroyed much of the city, starting on 6 October 1915, Belgrade fell to German and Austro-Hungarian troops commanded by Field Marshal August von Mackensen on 9 October of the same year. The city was liberated by Serbian and French troops on 1 November 1918, under the command of Marshal Louis Franchet d'Espèrey of France and Crown Prince Alexander of Serbia.