Straight from the legends…
When the young lad Merlin was brought before King Vortigern he asked: “why have your messengers brought me here?”
“My wise men,” answered the king, “have advised me to sacrifice a fatherless boy on the grounds which my tower is to be built. Unless I do this I shall be unable to build it.”
“Command the wise men here, and I will prove that they have lied to you,” the lad replied.
The king, amazed by the boy’s words, commanded the wise men to come. When they appeared before him, Merlin began to question them.
“Who told you to tell the king to sacrifice me? Speak, and tell us straightforwardly who told you to do this.” When they didn’t answer him, he turned to the king and said: “I will shortly explain everything to you, but first I wish to question your wise men.”
Then turning again to the wise men he continued, “Tell us. What is it that lies hidden beneath the foundation, for there is something that will not allow it to be built?” The wise men were filled with fear, and did not speak. So Merlin continued:
“My lord the king, tell your workmen to dig the soil, and you will find a pool beneath it that prevents that building of the tower.”
When this was done, a pool was found under the earth, just as the lad had said.
Now command, O king, that the pool be drained, and in the bottom you will find two sleeping dragons.”
The king, believing Merlin’s words, commanded that the pool should be drained. As the king sat and watched, the two dragons woke up and came forth from the pool, the one white and the other red.
When the dragons came near to each other, they began to struggle with each other in a fiery combat. The white dragon, the stronger of the two, drove the red dragon to the edge of the pool. But the red dragon fought back, falling on his enemy fiercely, and forcing the white dragon back into the pool. This happened three times before the red dragon regained his strength and drove the white dragon away from the pool. He pursued the white dragon until both disappeared.
“The meaning of this mystery is clear,” began the lad. “The pool is a symbol of this country, and the two dragons are two races who fight for its possession. The red dragon signifies Britain, who will be oppressed for a long time by the white dragon, which signifies the Saxons. In the end, the Britons will arise and drive forth the Saxons beyond the sea, where they came from, just as the red dragon drove the white dragon from the pool.”
-from “Heroes of Old Britain: Retold from Geoffrey of Monmouth” (abridged by @bbcMerlinJunkie)