In Romeo and Juliet, who causes the fight in Act 1 Scene 1?

The
conversation betweenand , two servants of the house of Capulet, make it clear that they are out
to provoke the Montagues, their masters' lifelong enemies, into a fight. They speak about how
much they hate the Montagues and how they would fight them in an encounter. When they see two
Montagues, Abraham and , approaching, they decide to start a quarrel:

GREGORY
I will frown as I pass by, and
let them take it as
they list.

SAMPSON
Nay, as they dare. I will bite my thumb at
them;
which is a disgrace to them, if they bear it.

Sampson is clearly intent on daring the two men and decides to make an insulting
gesture which, he knows, would offend the two and draw them into a confrontation. He bites his
thumb, which is a vulgar indication of his disrespect for the two men. Abraham questions his
action and Sampson at first denies the gesture but then states that he is, after all, biting his
thumb. Gregory then asks Abraham if he is quarreling, an accusation that he denies.

Sampson, for his part, informs Abraham that he is for him, since he is employed by a
master similar in status to his. He will, therefore, not stand back and tolerate any abuse.
Abraham's response, "no better," is a clear insult (he claims that his master is
better than theirs or that they are equal in status). The two Capulet servants then insist that
he acknowledge the superiority of the house they serve, and when he refuses and tells them that
they are lying, Sampson then challenges them to a duel:

SAMPSON
Draw, if you be men. Gregory, remember thy swashing
blow.

Sampson has probably just noticed , also of the
Capulet house, on his way, and is obviously emboldened. The men start fighting and(a Montague)
steps in and attempts to stop the fight. Tybalt then arrives and threatens him. The fight soon
spreads and a number of supporters from both houses join in the fray, including lords Montague
and Capulet.

It should be obvious that the chief perpetrator in this incident
is Sampson. He took the lead throughout and was intent on creating a fracas, continuously
confirming with Gregory whether they would have the law on their side and then succeeding in his
nefarious purpose.