The dual themes of ambition and betrayal run throughout Shakespeare's Macbeth, and both are foreshadowed early in the play when the Thane of Cawdor betrays King Duncan, and Duncan gives the title of Thane of Cawdor to Macbeth as a reward for Macbeth's victories in battle.
Duncan is wholly unaware of the irony of the situation.
DUNCAN. There's no art
To find the mind's construction in the face:
He was a gentleman on whom I built
An absolute trust. (1.4.13–16)
At this moment, Macbeth enters the scene. Macbeth has been utterly loyal to Duncan and has represented himself well, if not heroically, in many battles on Duncan's behalf.
Even when the Witches prophesy that Macbeth will be king, Macbeth rejects his previous ambitions for the throne and seems content to wait and see what develops over time.
MACBETH. [Aside.] If chance will have me king, why, chance
may crown me
Without my stir. (1.3.155–157)
It's in act 1, scene 4, however—when Duncan formally declares his son Malcolm as his heir and successor...
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