How does Iago cause Cassio to lose his position as lieutenant?

In Giuseppe Verdi’s opera Otello, Iago orchestrates Cassio’s loss of his position as lieutenant through a calculated scheme of manipulation and intoxication.1

Iago, who harbors deep resentment toward Cassio for receiving the promotion he desired, invites Cassio to drink during a victory celebration in Cyprus. Despite Cassio’s initial resistance, Iago successfully plies him with alcohol until he is intoxicated. Once Cassio is in this vulnerable state, Iago encourages Roderigo to provoke him into a fight.1

The resulting brawl, during which Cassio wounds the former governor Montano, draws the attention of Otello. Upon discovering that his lieutenant has caused a public disturbance and injured a man while drunk, an infuriated Otello immediately demotes Cassio. Iago’s plan is successful, as it removes his rival from his post and creates an opening for Iago to further manipulate Otello’s trust.12