In act 1, scene of Shakespeare's Romeo and Juliet, Benvolio tries to separate servants of the Capulets and the Montagues who have begun a brawl in the streets of Verona.
BENVOLIO. Part, fools!
He beats down their swords.
Put up your swords. You know not what you do. (1.1.59-60)
Not only is Benvolio unsuccessful in separating the combatants, but Benvolio himself becomes involved in a sword fight with Tybalt.
In act 3, scene 1, Benvolio steps in between Tybalt and Mercutio who appear ready to cause another brawl in the streets.
BENVOLIO. We talk here in the public haunt of men.
Either withdraw unto some private place
And reason coldly of your grievances,
Or else depart. Here all eyes gaze on us. (3.1.49-52)
Benvolio makes no further attempts to act as peacemaker in the scene. In fact, Benvolio does nothing, even when Romeo calls out to him to intervene between Tybalt and Mercutio later in the scene.
ROMEO. Draw, Benvolio; beat down their weapons. (3.1.84)
In the confusion of Romeo stepping in between Mercutio and Tybalt, Mercutio is fatally wounded, and Benvolio has nothing to say except to ask Mercutio, "What, art thou hurt?" (3.1.92). Tybalt runs away. Benvolio helps Mercutio out of the street, and returns a minute later to say that Mercutio is dead. Tybalt returns—"Here comes the furious Tybalt back again," says Benvolio (at 3.1.122)—but Benvolio does nothing to prevent or intervene in a sword fight between Tybalt and Romeo that results in Tybalt's death.
In the role of peacemaker, Benvolio is notably, and, as regards Mercutio and Tybalt, fatally ineffective. Benvolio is far more effective at cleaning up the mess left by the two deadly sword fights. He first orders Romeo to run away, to avoid Romeo being arrested by the Prince's men and condemned to death by the Prince.
He then lies to the Prince about what occurred by blaming Tybalt for the fight with Mercutio, even though Mercutio instigated the fight and was at least equally at fault for the fray. Benvolio also tells the Prince that he stepped in to try to prevent the fight between Romeo and Tybalt—which he didn't—but he neglects to mention that he told Romeo to run away.
BENVOLIO. ...And, as he [Tybalt] fell, did Romeo turn and fly.
This is the truth, or let Benvolio die. (3.1.179-180)
There's no one left alive or otherwise still on the scene to contradict Benvolio, so he's fairly safe in swearing on his own life that his story is true. In any event, Benvolio's version of events results in Romeo being banished from Verona by the Prince instead of being executed for Tybalt's death.
It's interesting to note that Benvolio doesn't appear again in the play, and his name isn't even spoken again, although some directors bring Benvolio back with the other characters who assemble at Juliet's tomb at the end of the play. Interesting, too, is that in the first quarto version of Romeo and Juliet, published in 1597—often called the "bad quarto," much like the first quarto of Hamlet is called the "bad quarto"—Benvolio dies.
. [to the Prince
] Dread Sovereign, my wife is dead tonight,
And young Benvolio is deceased too... (Q1, 5.3.233-234)
No cause or reason is given for Benvolio's death, but this might explain why he doesn't appear again after act 3, scene 1 in subsequent versions of the play. Then again, perhaps it's just a "bad quarto."