The line of succession in William Shakespeare's Macbeth acts as one of the major catalysts for Macbeth's deceit. After proving his valor in the initial battle that opens the play, Macbeth, the Thane of Glamis, is given the additional title of Thane of Cawdor. When he is presented with this honor, Ross tells him it is "for an earnest of a greater honor" (1.3.109). The combination of the Weird Sisters' prophecies and Ross's choice of words helps Macbeth to convince himself that his newly acquired position is simply another step toward his eventual kingship.
In an aside later in the same scene, Macbeth even refers to the promotion as one of two "happy prologues to the swelling act / Of the imperial theme" (1.3.141-142). However, in the very next scene King Duncan clearly lays out his plans for the succession, stating, "[w]e will establish our estate upon / Our eldest, Malcolm, whom we name hereafter / The Prince of Cumberland" (1.4.43-45). The title of Prince of Cumberland . . . Here Macbeth is taken aback, and allows the reader a glimpse into his plans, stating in an aside:
The Prince of Cumberland! That is a step
On which I must fall down or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires.
Here Macbeth shows that, despite the established line of succession, he plans to find a way to become king and fulfill the prophecies of the Weird Sisters. After he murders Duncan in act 2, scene 2, Malcolm and Donalbain, fearing for their own lives, flee Scotland. This makes it easy for Macbeth "The Great War Hero" to easily step in and take the throne for himself. By the beginning of act 3, Macbeth is already firmly in place as king, and Scotland has already started to decline.