How is the theme of usurpation central to The Tempest in Act 1?
Usurpation is key to Act I of this play, as Prospero explains to his daughter how they ended up on this enchanted island and in addition how Prospero himself seized power. Clearly in Act I scene 2, Prospero is explaining to Miranda something of her history both for her benefit and the audience's benefit. Prospero begins by telling Miranda how his own brother usurped his role as Duke of Milan and seized power for himself. However much Prospero bemoans how cruel his brother was to seize power from him in such a way, it is clear that he himself does exactly the same thing when he and his daughter reach the island. Note what he reminds Ariel of whilst his daughter sleeps and he talks about Sycorax, his former mistress:
This blue-eyed hag was hither brought with child,
And here was left by th'sailors. Thou, myslave,
As thou report'st thyself, was then her servant;
And for thou wast a spirit too delicate
To act her earthy and abhorred commands...
Sycorax and her son Caliban, oddly mirroring Prospero and Miranda, were likewise sent to the island and exiled there, and Sycorax used her magical arts to seize power and enslave Ariel to her own purposes. It is Prospero who usurped her power when he arrived at the island and freed Ariel, enslaving both Ariel and Caliban into his service. As much as Prospero feels hard done by through his own loss of power, he is quick to usurp the power of another.