In the military, soldiers respect rank. In Othello, Shakespeare positions a black man as a leader which would be disregarded in the society at the time, but because he is a significantly powerful military leader, he is the character to be respected and revered. This status of the Moor positions the rest of the characters in the play.
As the play opens, Iago is strategically moving to get a position just below Othello, but Othello gives the position to Cassio. This incites the main action of the play because Iago's jealousy of Cassio's appointed position causes Iago to play Cassio against Othello. As right-hand man, Cassio is out to war with Othello when an ironic storm arises bringing Cassio quickly back to shore. Cassio is extremely worried about Othello during the storm. By the time all is well and Othello is back, Iago is already pitting Cassio against Othello by getting Cassio drunk and putting ideas in Othello's head about Cassio and Desdemona having an inappropriate relationship. Thus throughout the storyline, Iago uses evidence and militant strategy to continue this division between Cassio and Othello all because he was mad and wanted the position Cassio has.