The history of the Welsh flag
The Welsh people were the original Britons, who lived in Britain before the Romans arrived, but Wales as it currently exists dates from the Anglo-Saxon invasion which took place from around 450 AD onwards, and pushed the Britons westwards.
The Welsh flag however is much younger than that.
Dragons are a long-standing motif for the Brythonic peoples (the walled City of London, which became a sanctuary for the Britons during the anglo-saxon invasion, also has a dragon on it's emblem).
But the flag with the green and white background is Henry Tudor's banner.
Who was Henry Tudor?
He was the grandson of a Welshman, Owen ap Maredudd apTudor who seduced Katherine of Valois, Henry V's widow, mother of Henry VI and daughter of the Charles VI of France. It is said that Owen met her at a party, where he was drunk and she took a fancy to him. The Welsh had no rights at the time, but Katherine persuaded Parliament to pass a special act in 1432 giving him the rights of an English lord, and then the two of them retired to the country and set up house together and had six children together. These children were half-brothers to the child king Henry VI and were raised with him.
Katherine de Valois died in 1437 after a serious illness, and her enemies arrested Owen Tudor. However, when Henry VI became an adult, he freed Owen Tudor, and took a hand raising his half-brothers and gave them earldoms. One of these half-brothers, Edmund Tudor, married Margaret Beaufort, who was descended from an illegitimate line of the House of Lancaster, and they had a son Henry Tudor, in Wales.
However, the House of York seized the throne, murdering both Henry VI and his step-father Own Tudor. Thus there was no-one left of the House of Lancaster, apart from Henry Tudor who had no legal claim to the throne.
But he was determined to seize the throne anyway. From his exile amongst the Breton-speaking people of Brittany (who are descended from Britons who emigrated to Brittany during the anglo-saxon invasions), he plotted how to challenge the Plantagenet King Richard III for the throne.
In 1485 he landed on the Welsh coast with some Breton mercenaries, and proceeded through the towns of Wales collecting supporters and armed volunteers. The Welsh nobles were enthusiastic - they felt that if Henry Tudor seized the throne, at last their rights would be restored.
Henry made much of his Welsh ancestry, and even visited a famous Seer, to ask him whether he was "Y Mab Darogan", the "son of prophecy" who would free the Welsh from oppression. The story goes that the seer was unsure, but his wife advised him, "Say yes, what have you got to lose, if Henry Tudor loses the battle he is unlikely to return here". So he said yes.
The Welsh were excited by this, and one of the armed volunteers took Henry's green and white banner and stiched a Welsh Dragon on it. And the Welsh flag was born and flew over the battlefield at Bosworth Field where Henry challenged and defeated Richard III and crowned himself king.
Did the Tudors free the Welsh from oppression?
Henry Tudor, who became King Henry VII, was slow to keep his promises. He was more interested in acquiring money and milking England for tax, while neglecting Wales.
However his son Henry VIII passed an Act of Union between England and Wales in 1536, uniting the two legal systems, so that the English and Welsh had the same rights, and giving the Welsh representation in Parliament for the first time.
And Elizabeth Tudor, also known as Elizabeth I, passed a very important law ordering the bible and common prayer book to be translated into Welsh, and ordering church services in Wales to be conducted in Welsh. It was an important cultural moment. Up until then the Welsh had their church services in either Latin or English. Elizabeth's Welsh bible was of a high quality, and it's style influenced Welsh literature. It is believed that without the Welsh bible the language would have died out in the way Scot-Gaelic and Irish-Gaelic did. As it is, Welsh is the only Celtic language that is thriving (Breton got eliminated by the French revolutionaries, Cornish died, Scots-Gaelic is only spoken by 1.1% of Scits and Irish Gaelic is spoken by just 7% of Irish as their first language).
So the Tudor dynasty really did save Wales, and they now fly Henry Tudor's banner as the official Welsh flag.