The history of the Saltire

In 834AD, a Pictish army under King Angus (or Hungus) Macfergus, High King of Alba, along with a force of Scots under King Eochaid of Dalriada, was preparing to go into battle with a force of Northumbrian Angles under their leader Athelstane, for the control of the Lothians which was at that time under Northumbrian rule.
The night before the battle, Angus knelt in prayer and is said to have had a vision of Saint Andrew who promised him victory on the following day.
In the morning, just before the battle commenced, a large white diagonal cross, like that on which Andrew had been martyred appeared in the blue sky.
The sight of this cross was taken as a good omen and filled the army with renewed strength. Angus and his force went into battle and gained a great victory over Athelstane, and from then on, the Saltire became the national flag of Scotland.

The Saltire is probably the oldest flag in Europe.