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Iago learns of Cassio's weakness against alcohol. With this knowledge, he constructs a brilliant plan that will force Othello to relieve Cassio of his duties as lieutenant. The night that Cassio is left on guard by Othello, one of the most important moments in Cassio's career as lieutenant, is the night Iago chooses to unleash a masterful plan of destruction and manipulation on the life of Cassio. Iago first joins Cassio on guard in order to create the idea that he is a loyal comrad and friend to Cassio. Iago acts as though he is concerned with Cassio's well-being when in response to Cassio's crippled state, he exclaims "Marry, God forbid!" (2.3.280). With this newly established sense of genuine caring Cassio believes Iago feels towards him, Iago is now capable of suggesting actions Cassio can take in order to feel better without suspicion. Iago uses this trust to convince Cassio to drink, for "wine is a good familiar creature, if to be well used" (2.3.328-329) Iago claims. Roderigo can now easily provoke Cassio in his inebriated state. Cassio falls right into Iago's trap, first engaging in a brawl with Roderigo, then with Montano. Due to Cassio's foolish behavior, Othello has no choice but to strip Cassio of his lieutenancy. .
Iago devises an ingenious plan that creates the suspicion of an affair between Desdemona and Cassio through the eyes of Othello. Initially, Iago does not directly state that there is an affair between Desdemona and Cassio, but he does insinuate the possibility of this affair. Iago first states "For Michael Cassio, I dare be sworn I think that he is honest" (3.3.144-145). He then continues to say "Men should be what they seem; or those that be not, would they might seem none" (III, iii, 147-148). In stating this, Iago is insinuating the idea that Cassio is dishonest. Later in this conversation, Iago claims in regards to Desdemona that "her will, recoiling to her better judgement, may fall to match you with her country forms and happily repent" (3.
