There is a supposed rivalry between Cassio and the play's villain, Iago. Iago claims to resent Cassio because Othello chose Cassio rather than Iago as his lieutenant, in spite of the fact that Cassio has no practical knowledge of battle.
Iago alleges a rivalry with Cassio in the play, claiming that he dislikes Cassio due to Othello's selection of Cassio rather than Iago for lieutenant. Despite Cassio's lack of combat knowledge, this decision has caused Iago's resentment.