How does Macbeth earn his title of Thane of Cawdor?

The Scottish army is at war with the Norwegian army. Duncan, king of Scotland, meets a captain returning from battle. The captain informs them of Macbeth and Banquo's bravery in battle. He also describes Macbeth's attack on the castle of the treacherous Macdonald, in which Macbeth triumphed and planted Macdonald’s head on the battlements of the castle. The Thanes of Ross and Angus enter with the news that the Thane of Cawdor has sided with Norway. Duncan decides to execute the disloyal thane and give the title of Cawdor to Macbeth.