The immediate precipitant of World War I was Austria-Hungary’s July ultimatum to Serbia, demanding an investigation of the assassination of Archduke Franz Ferdinand. When Serbia refused to accede to the ultimatum in full, Austria-Hungary declared war on 28 July, beginning the shelling of Belgrade the following day. Although heavily committed with Germany to defending its Russian front, Austria-Hungary had a huge advantage over Serbia in available troops and materiel, and sought a rapid victory by invading Serbia on 12 August. On home ground, in rough terrain, the Serbs overcame the enemy near Sabac and forced their retreat. Emboldened, the Serbs attempted a counteroffensive by crossing into Bosnia. The annexation of Bosnia and Herzegovina, with its large Serb population, had been the provocation that led ultimately to the assassination of Archduke Franz Ferdinand, but Serbia had insufficient numbers to make a decisive strike and were repulsed with heavy losses.