Battle of the First World War. After the defeat of their first invasion, the Austrians mounted another attack on Serbia, crossing the River Drina on 7 September. The Serbs rushed their army back from a brief invasion of Austrian Bosnia, but despite ten days of fierce fighting were unable to push the Austrians out of their bridgeheads, and withdrew to form another line of defence before Belgrade, which fell to the Austrians on 2 December.