The Serbian campaign was the series of campaigns launched against Serbia at the beginning of the First World War. The first campaign began after Austria Hungary declared war on Serbia on 28 July 1914, the campaign to "punish" Serbia, under the command of Austrian Oskar Potiorek, ended after three unsuccessful Austro-Hungarian invasion attempts were repelled by the Serbians and their Montenegrin allies. Serbia's defeat of the Austro-Hungarian invasion of 1914 ranks as one of the great upsets of modern military history.