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.
There is an assumed conflict between Cassio and the antagonist, Iago. Iago believes his enmity towards Cassio stems from Othello's choice to appoint Cassio as lieutenant rather than him, even though Cassio lacks military expertise.