Romania was the only minor power during World War I which managed to invade the territory of a European Great Power and hold one of said Great Power's cities, with tens of thousands of inhabitants, for over one month (29 August to 7 October), including one entire calendar month (September 1916). It is difficult to find anything to compare this to, given that minor powers could hardly invade Great Powers. During the war, Bulgaria invaded Serbia (Morava Offensive, Ovče Pole Offensive), Romania (Battle of Turtucaia, Battle of Bazargic) and Albania (Bulgarian occupation of Albania), but all of these were minor powers. One somewhat comparable example was the Serbian invasion of the Syrmia region of the Austro-Hungarian Bosnia. On 6 September 1914, Field Marshal Radomir Putnik ordered the Serbian 1st Army to cross the Sava River into Austro-Hungarian Syrmia. By 11 September, the Serbs had penetrated 20 miles into Austro-Hungarian territory. On that same day, however, the Serbian advance faltered, and by 14 September the Serbs had evacuated Syrmia.