At the beginning of the first scene in Shakespeare's Othello, Iago is complaining bitterly to Roderigo that Othello has chosen someone other than Iago to be his lieutenant, his second-in-command.
Iago tells Roderigo that he had arranged for well-placed Venetian citizens ("three great ones of the city") to speak to Othello on his behalf. Othello wouldn't see them, Iago says. Othello made overblown excuses to avoid seeing them and told them that he has already chosen a lieutenant: Michael Cassio.
Iago provides information about his own qualifications for the position by explaining Cassio's shortcomings to Roderigo.
Iago says that Cassio is a Florentine, not a Venetian, implying that Cassio has mixed loyalties to Othello and Venice.
Iago considers his own military experience far superior to Cassio's. Iago says that Cassio has no experience commanding men on the field of battle and implies that Cassio can't even control his own wife.
Cassio is schooled ("a great arithmetician"), whereas Iago is not. Iago mocks his schooling and military training as being limited to a theoretical study of Roman military exploits—"the bookish theoric, / Where in the toga'd consuls can propose / As masterly as he."
Iago says that Cassio talks a good game, militarily, but has no experience to back it up.
In contrast, Othello has seen Iago's military skills firsthand, in battles at Rhodes and in Cypress, as well as other campaigns in Christian and non-Christian countries alike. Even though Iago has much more training and experience than Cassio, Othello chose Cassio, whom Iago calls the bookkeeper ("this contercaster"), implying that he knows nothing but debits and credits.
Iago finds himself stuck, seemingly forever, as "his Moorship's ancient": Othello's ensign and flag-bearer.
Iago seems to be resigned to Othello's decision and to the current state of affairs in the military.
IAGO. Why, there's no remedy. 'Tis the curse of service,
Preferment goes by letter and affection,
And not by old gradation, where each second
Stood heir to the first. (1.1.34–38)
However, Iago is by no means as accepting of the situation as he says.
RODERIGO. I would not follow him then.
IAGO. O, sir, content you.
I follow him to serve my turn upon him... (1.1.41–43)
In time, Iago will take revenge on Cassio and Othello for Othello's choice of Cassio as his lieutenant.