Years before Kenneth MacAlpin succeeded in uniting the Kingdoms of Dalriada and Pictland, King Angus of the Scots was invading Lothian, at that time under Northumbrian control. However, the Scots encountered a larger force led by the Northumbrian King Athel.
The Scots army was forced to retreat, and were cornered near the Peffer Burn. Angus's men were severely outnumbered, so the King and his companions (which included Kenneth) prayed to God and the Saints that they would be delivered from the hands of their enemies. Soon after the fighting began, the cross of Saint Andrew appeared in the clouds above the field. Emboldened by this sign from above, the Scots went on to victory, even against such overwhelming odds.
Seeing that the battle was turning in favour of the Scots, King Athel attempted to build a ford and escape across the burn. However, he was slain while trying to cross, and that place was forever known as Athelstaneford.
When Kenneth united the Scots and Picts, creating the Kingdom of Alba, he remembered the Scots' triumph over the Northumbrians at the hands of Sint Andrew, and so Andrew became patron Saint of Scots. The battle is remembered in the Scottish flag, which depicts a white Saltire cross against the blue sky. It is also commemorated in the celebration of St Andrew's day, which is held each year on the 30th of November.