How the fiendish Iago plays upon Othello's jealous heart until one sees that: Trifles, light as air, Are to the jealous confirmations strong As proofs of holy writ.
Iago bitterly resents a slight he feels Othello has put upon him.

With his large, generous, unsuspicious nature, Othello never dreams of such a thing; he trusts Iago as his intimate friend, and thus gives the crafty fiend the oportunity he desires to put the Moor Into a jealousy so strong That judgment cannot cure ...
Make the Moor thank me, love me, reward me, For making him egregiously an ass And practicing upon his peace and quiet Even to madness.
Othello gives his wife, Desdemona, a rare handkerchief.

Iago urges his own wife, who is Desdemona's maid, to pilfer this and bring it to him.
When he gets it, he leaves it in Cassio's room.

Cassio was an intimate friend of Othello's, one, indeed, who had gone with him when he went to woo Desdemona, and who, by Iago's machinations, had been suspended from his office of Othello's chief lieutenant.

To provoke Othello's jealousy Iago now urges Desdemona to plead Cassio's cause with her husband, and at the came time eggs on Othello to watch Cassio: Look to your wife; observe her well with Cassio; Wear your eye thus, not jealous nor secure.
I would not have your free and noble nature Out of self-bounty be abus'd; look to 't.
Thus he works Othello up to a rage, and yet all the time pretends to be holding him back: I do see you're mov'd; I pray you not to strain my speech To grosser issues nor to larger reach Than to suspicion.
Iago leaves the handkerchief in Cassio's room, at the same time saying: The Moor already changes with my poison; Dangerous conceits are in their natures poisons, Which at the first are scarce found to distaste, But with a little act upon the blood, Burn like the mines of sulphur.
And as he sees the tortures the jealous worries of the Moor have already produced in him, he exultingly yet stealthily rejoices: Not poppy, nor mandragora, Nor all the drowsy syrups of the world, Shall ever medicine thee to that sweet sleep Which thou hadst yesterday.
Well might Othello exclaim that he is "Set on the rack." Each new suspicion is a fresh pull of the lever, a tightening of the strain to breaking point, and soon his jealousy turns to the fierce and murderous anger Iago hoped it would: Like to the Pontic sea, Whose icy current and compulsive course Ne'er feels retiring ebb, but keeps due on To the Propontic and the Hellespont, Even so my bloody thoughts, with violent pace, Shall ne'er look back, ne'er ebb to humble love, Till that a capable and wide revenge Swallow them up.
Thus was he urged on, worried by his jealousy, until, in his bloody rage, he slew his faithful wife.