The theme of usurpation runs throughout Shakespeare's The Tempest and is central to the play.
Interestingly, the term "usurp" itself appears only once in the play: when Prospero accuses Ferdinand of usurping the name of Ferdinand's father, the king of Naples, who Ferdinand assumed had been killed in the shipwreck. Prospero also accuses Ferdinand of being a spy:
PROSPERO: Thou dost here usurp
The name thou ow'st not; and hast put thyself
Upon this island as a spy, to win it
From me, the lord on't. [1.2.540–543]
Nevertheless, the theme of usurpation—political, social, and personal—underscores the action of the play, as well as the related themes of betrayal, natural order, and revenge.
As Prospero explains to his daughter, Miranda, in act 1, scene 2, twelve years ago Prospero was the Duke of Milan, until his brother, Antonio (with the help of the king of Naples) betrayed Prospero, usurped his dukedom, and seized his power.
PROSPERO: The King of Naples, being an enemy
To me inveterate, hearkens my brother's suit;
Which was that he, in lieu o'th’ premises
Of homage and I know not how much tribute,
Should presently extirpate me and mine
Out of the dukedom, and confer fair Milan,
With all the honours, on my brother . . . [1.2.142–148]
Antonio sent Prospero and Miranda into exile on a leaky excuse for a boat, which landed on the shore of their island.
PROSPERO: . . . In few, they hurried us aboard a barque,
Bore us some leagues to sea, where they prepared
A rotten carcass of a butt, not rigged,
Nor tackle, sail, nor mast—the very rats
Instinctively have quit it. . . .
Here in this island we arrived . . . [1.2.169–173, 202]
Good fortune brought the usurpers to the island:
PROSPERO: By accident most strange, bountiful Fortune,
Now my dear lady, hath mine enemies
Brought to this shore . . . [1.2.210–212]
Here, Prospero can use his magic to confuse and confound his enemies and effect his own kind of revenge on them for usurping his dukedom many years ago.
PROSPERO: [to Antonio] For you, most wicked sir, whom to call brother
Would even infect my mouth, I do forgive
Thy rankest fault, all of them, and require
My dukedom of thee, which perforce I know
Thou must restore. [5.1.145–150]