How does Shakespeare portray Iago's power in Othello?

Shakespeare portrays Iago as the most powerful person in the play to the extent that his manipulations and schemes propel the deadly conflict in Othello.

Although Othello technically has more power than Iago—Othello is a general, while Iago is a soldier—Iago exhibits his superior powers of manipulation and subterfuge after Othello picks Cassio and not him to be his lieutenant. Iago gains power over Othello by planting the idea in his mind that Desdemona, his wife, is having an affair with Cassio. “Did Michael Cassio, when you woo'd my lady, / Know of your love?” Iago asks Othello ever so slyly. He’s pretending to be on Othello's side, but really he’s planting the seeds for his toxic plan. The nefarious stratagem involves the other characters in the play, including Roderigo, who’s in love with Desdemona, and Emilia, Iago’s wife.

Emilia plays a crucial role because she steals the treasured handkerchief for her husband so he can frame Cassio. Iago also makes Othello think Cassio is bragging about how much Desdemona desires him when actually he’s talking about Bianca, a sex worker. Iago’s complex and vicious revenge plot pilots the play's characters and key events. Iago is the most powerful person in the play because, without his cruel wrath, Othello would have no reason to believe Desdemona was cheating on him. As such, he wouldn’t have killed her or himself.

How does Othello express his understanding of his power in Shakespeare's Othello?

In Shakespeare's Othello, Othello understands that he has held great power in the military, and he calls on this power in order to have himself heard after it becomes clear to all that he has murdered his wife.  Near the very end of the play in Othello's last few lines, he tells the men that "[he has] done the state some service," and rather than haul Othello off as a prisoner, the men allow him his last few words.  Othello is thus able to ask the men to, in the future, speak the truth about him and his character and about how much he loved Desdemona yet was tricked into confusion.  The men hear Othello's words out of respect for the power that he has held as their leader in the military, and even Cassio, who was wronged by Othello, is able to recognize that Othello was a man "great of heart."