Pomp and circumstance
I'm Othello. I'm a general in the Venetian army, and everyone seems to think I'm honest, loyal, and levelheaded. But, by the end of the play, I turn irrational, violent, and jealous. And you know what I think?
I had been happy, if the general camp,
Pioners and all, had tasted her sweet body,
So I had nothing known. O, now, for ever
Farewell the tranquil mind! farewell content!
Farewell the plumed troop, and the big wars,
That make ambition virtue! O, farewell!
Farewell the neighing steed, and the shrill trump,
The spirit-stirring drum, the ear-piercing fife,
The royal banner, and all quality,
Pride, pomp and circumstance of glorious war!
And, O you mortal engines, whose rude throats
The immortal Jove's dead clamours counterfeit,
Farewell! Othello's occupation's gone! (3.3.342-354).
Who Said It and Where
Iago hates Othello. Why? Well, for starters, Othello (a general in the Venetian army) chose another man, Cassio, to be his lieutenant. Iago really wanted the job and can't understand why he didn't get it. To be fair, he's also just angry in general. Oh, and did we mention he's racist? Yep, Iago's not really a stand-up guy no matter which way you slice it.
He decides to get revenge by convincing Othello that Cassio is having sex with Othello's wife, Desdemona. It's totally untrue of course, but that doesn't stop him from spreading the lie. See, Othello's no dummy. He'll see right through an outright attack on Cassio. So Iago's gotta go the subtle route.
Iago decides to plant the seed and let Othello fill the rest in on his own. He essentially says, "OK, if you promise you won't be jealous, you should watch Desdemona with Cassio, but objectively, and not out of jealousy." Iago says he knows that women of Venice are promiscuous, and though heaven knows their little exploits, their men don't.
Iago adds (to help with the objectivity bit) that Desdemona did deceive her father in order to marry Othello. He's implying, as Brabantio did earlier, that Desdemona is not to be trusted, as she is a woman, and therefore a liar. That's how that works, right? Basically, Hamlet could get together with these guys and have a big anti-women party. (Frailty, thy name is lying, scheming woman!)
His number one goal in this scene is to let Othello brood over the possibility that Desdemona is cheating on him, a supposedly undesirable black man. As Othello's busy wondering why he ever got married in the first place, Iago tells Othello not to think about it too much—it's probably nothing, he's probably overreacting, etc.
Othello says he'd been happy in the good old days when he was just a general in the army and didn't have to worry about women and all that jazz. It was such a simpler time. All there was to think about was the ceremony and spectacle of war. Ah, the simple life.