Through the course of this tragedy, Cassio's fortunes change considerably. In Act I, he is Othello's loyal and trusted lieutenant. In Act II, he is Othello's loyal friend in Cypress and respectful admirer of Desdemona but in Act II, Scene III, is manipulated to fight Roderigo, hitting him and Montano, and consequently losing his position as Othello's "lieutenant".
Iago's wife, Emilia, tells Cassio that Othello would like to reinstate him as his lieutenant but the fact that Cassio's fight is public news, prevents Othello from doing this immediately.
Iago: Othello's ancient, a position below lieutenant. Perhaps Shakespeare's most evil figure, not for his treachery which is arguably surpassed by Macbeth (who kills his king in his sleep while staying at his castle as a trusting guest), but for the manner in which he effortlessly manipulates all those around him to do his bidding (kill Cassio, destroy Othello, discredit Desdemona's virtue) by taking advantage of their trust and using his victim's own motivations (Roderigo's desire for Desdemona, Cassio's desire to be reinstated) and weaknesses (Othello's pride, Cassio's impaired judgment whilst drunk), to achieve his ends.
Iago's character is complex, but in Act I, Scene I, where he describes his disgust at being overlooked for Othello's lieutenant, we can see that a primary motivation for Iago's skillful manipulations was revenge and anger; revenge for Cassio replacing him, anger that Othello overlooked him. Thus it can be seen that Iago's manipulations are driven by a basic desire to avenge those who hurt him but also to gain what he believes is his, indeed Iago's suggestion that Cassio and Desdemona are having an affair earns Iago Othello's trust and the position as his lieutenant in Act III, Scene III.
Finally in Act V, Cassio first fights off an ambush by an Iago manipulated Roderigo, then is stabbed by Iago in the dark to later outlive Othello, Desdemona and Emilia and be placed in charge of the now exposed and captive Iago.
Next Iago suggests to Othello that Cassio will "blab" or gloat to others about his conquest of Desdemona before telling Othello that Cassio boasted to him that he did indeed sleep with Desdemona.
Iago fans the flames of Othello's distrust and fury with Desdemona's supposed "infidelity" by first suggesting Desdemona shared her bed with Cassio and then that her giving away the handkerchief is no big deal when Iago knows exactly how hurtful to Othello, giving away this sentimental gift is.
Iago is pragmatic. We see this in the manner in which he uses opportunity to aid his plan to hurt Cassio and Othello. When Iago realizes Cassio's admiration of Desdemona, he immediately formulates a plan to use this to make Othello suspect adultery.
Though Iago appears to have the goal of misery for Othello and Cassio, he does not appear to have a specific detailed plan, he continually refers to using insights he makes about Cassio, Othello, Roderigo and Desdemona to further his plan in his asides in the play.
However being made lieutenant only satisfies his pride, his continuing with his plan to discredit Desdemona shows us that it is not enough for Iago to have what he believes is his, he must punish Othello for overlooking him in the first place by making Othello disbelieve and destroy his virtuous wife...
Cassio gives Othello's handkerchief, which he found, to his suspicious mistress Bianca who reluctantly starts to copy its patterning (presumably its strawberry motif / design) for him.
Cunning in the extreme, Iago nearly gets away with his plan; Othello does kill Desdemona, Iago is made lieutenant, but Cassio despite his attack in Act V, lives and he (Iago) is eventually caught and exposed.
In Act IV, Cassio is manipulated into talking about his girlfriend Bianca, unaware that Othello, hiding nearby assumes his hand gestures are descriptions of Desdemona.
Lodovico arrives, announcing that Othello is to return home and Cassio is to be the next Governor of Cypress. Desdemona's joy for Cassio enrages Othello, leaving Lodovico and Iago to wonder how much Othello seems to have changed and leaving poor Desdemona to wonder how she offended the man she truly loves...
In Act III, Iago is Othello's remorseful friend who hopes Desdemona's good words will reinstate him, unaware that they merely implicate him in Iago's plan to make him look like Desdemona's lover and Cassio also reveals himself to be a neglectful boyfriend to his mistress Bianca.