In Act 2, Scene 3 of Shakespeare’s Othello, Iago gets Michael Cassio drunk. This provokes a fight between him and Roderigo. Montano tries to break up the fight, but Cassio wounds him. As a result, Cassio’s reputation is ruined. Othello loses trust in him, as well, and strips him of his officer’s rank.
A feast is in progress in Cyprus. Othello has assigned the task of keeping watch to his lieutenant, Cassio. When Cassio and Iago are left alone, Iago convinces Cassio to drink some wine. The lieutenant reasonably refuses and explains that he is not a good drinker and has already had his share for the night. But Iago presses on, taunting Cassio, inviting some local revelers to join them and singing some drinking songs. This convinces the lieutenant, and he quickly gets very drunk and leaves.
Iago, now alone with Montano, essentially calls Cassio a drunk and wonders why Othello keeps him as a lieutenant. When Roderigo passes by, Iago suggests he go after Cassio, setting his plan in motion. Iago knew that Roderigo is in love with Desdemona, Othello’s wife. So he convinced him that Cassio was, too. He also tried to get Cassio interested in the woman, but that part of his plan failed. So no wonder that when the two drunk men met, they quickly came to blows.
Iago’s conversation with Montano is interrupted when Roderigo runs in, chased by a drunken Cassio. The two keep fighting, and Montano tries to step in and break up the fight, but Cassio attacks and wounds him. Roderigo, meanwhile, runs off to sound the alarm. Iago’s plans are paying off. When Othello comes in to investigate, he sees Montano’s bleeding and the state Cassio is in.
Othello deals with the fighting by condemning Cassio and stripping him of his rank. This devastates him, as he is more worried about his now ruined reputation than any bodily wound. Iago comforts him and ensures that the man gets so drunk that he remembers no details of the night. Think about this: he will not be able to blame Iago for getting him drunk. Even if he could, no-one would believe the man who got drunk when he was supposed to keep watch.
And so, Iago’s plans have advanced to a significant degree. First, he has had his revenge on Cassio. In Iago’s opinion, he was passed over for promotion in favor of Cassio, so he is resentful of him for that. Second, he has undermined Othello’s trust in his (now former) lieutenant. This has a side effect of feeding Othello’s self-doubt. How could he not know he was making a violent drunk his right-hand man? He is going to doubt his own decisions more now. How is that useful to Iago?
These developments will play into his ultimate plan of convincing Othello that his wife is cheating on him with Cassio. They are two of his most trusted people, and Iago’s plot has undermined his trust in one of them. What’s more, Iago’s speech to Othello after the fight earns him more of his trust. This means he has maneuvered himself closer to his target and will have more opportunities to betray Othello.