In Shakespeare's Othello, when Othello chooses Cassio for a promotion over Iago, this creates the central conflict of the play. It is for this reason that Iago decides to destroy Othello.
In this quote, Iago seethes with anger over Othello's choice, and later says he will serve "the Moor" to "turn on him." (This conflict is external: specifically, man vs. man.)
Nonsuits my mediators; for, “Certes,” says he,
“I have already chose my officer.” … (I.i.16-17)
O, sir, content you.
I follow him to serve my turn upon him… (42-43)
Roderigo has a conflict with Othello, for he wanted Desdemona for himself. Iago enlists Roderigo's help in waking Desdemona's father, to stir trouble in the house with news that Othello has taken Desdemona and married her. (This is man vs. man.)
Here is her father's house; I'll call aloud. (78)
Another conflict arises when Desdemona's father (Brabantio) arrives in a fury over his daughter's marriage to Othello. He insists that Othello must have put a spell on her, for why else would she marry such a man as he—unless she was "in chains of magic...bound?" (This is man vs. man.)
O thou foul thief, where hast thou stow'd my
Damn'd as thou art, thou hast enchanted her,
For I'll refer me to all things of sense,
If she in chains of magic were not bound (I.ii.76-80)
In order to get Cassio in trouble so he will lose his job serving Othello, Iago first gets Cassio drunk—even though Cassio tries to resist at first. Then Iago enlists the help of Roderigo to draw Cassio into a fight. (Both instances of conflict are man vs. man.)
I have drunk but one cup tonight, and that was craftily
qualified too, and behold what innovation it makes here.
I am unfortunate in the infirmity, and dare not task my
weakness with any more. (II.iii.34-37)
Iago notes that everyone wants to party:
What, man! 'Tis a night of revels, the gallants desire it. (38)
And then Cassio is tricked into fighting:
A knave teach me my duty! But I'll beat the knave
into a twiggen bottle. (II.iii.139-140)
Cassio is punished by Othello, removed from his position. (Again, this is man vs. man.):
Cassio, I love thee;
But never more be officer of mine. (241-242)
After Iago poisons Othello against Desdemona, Othello's mental state deteriorates, and he kills the innocent Desdemona. His proof is that Cassio had Desdemona's handkerchief in his possession; however, it was planted in his room by Iago. (This conflict is man vs. man.)
That handkerchief which I so loved and gave thee
Thou gavest to Cassio. (V.ii.54-55)
When Emilia, Desdemona's maid (and Iago's wife) realizes that Othello has murdered his wife, she tells everyone. Those gathered are appalled. (This conflict is man vs. society.)
I will not charm my tongue; I am bound to speak.
My mistress here lies murdered in her bed.
O heavens forfend!
Emilia decides to find why Othello did such a thing—it all goes back to Iago; she accuses him. (This conflict is man vs. man.)
O thou dull Moor! That handkerchief thou speak'st of
I found by fortune and did give my husband;
For often with a solemn earnestness,
More than indeed belong'd to such a trifle,
He begg'd of me to steal it. (V.ii.262-266)
Othello accepts the blame and kills himself. (This is internal conflict: man vs. self.)
Then must you speak
Of one that loved not wisely but too well… (392-393)