In accordance with the fosterage customs of the Highland clans, Alan Stewart and his brothers grew up under the care of their relative James of the Glen in Appin. His nickname, Breck, came from the Gaelic for "spotted", as his face bore scars from smallpox. Stewart enlisted in the British Army of George II in 1745, just before the Jacobite rising of that year. He fought at the Battle of Prestonpans, but deserted to the Highland Jacobites. He subsequently fought for the Jacobites, but after they were defeated at the Battle of Culloden, he fled to France, accompanying his commander and clan captain, Colonel Charles Stewart of Ardshiel (Ardshiel was not the chief of the Appin Stewarts, but took command in the absence of the chief). After joining one of the Scottish regiments serving in the French Army, Stewart was sent back to Scotland to collect rents for the exiled clan leaders and to recruit soldiers for the French crown.