When he assumed sole control of the Roman empire in 30 BC, the emperor Augustus was faced with a pressing strategic anomaly. Although Rome had subjugated all Gaul up to the Rhine and much of Illyricum, the Alpine region which separated these possessions from Italy and from each other remained outside Roman control and in the hands of independent mountain tribes. These were warlike and troublesome, alternately attacking and robbing transient Roman troops and supply convoys or exacting exorbitant tolls from them for the privilege of using the key Alpine mountain passes e.g. the Salassi, who reportedly ambushed Julius Caesar on one occasion by hurling rocks on his army, charged Messalla extortionate fees for supplies and forced the escaping murderer of Caesar Decimus Brutus to pay a toll of one denarius per man to allow his army to cross the Great St Bernard Pass in 43 BC. That Rome's overland communications with its transalpine territories should be thus held to ransom was no longer tolerable, especially as Augustus was intent on advancing the Roman sphere of control as far as the Danube river.