One might suggest, at first glance, that all the brothers conspired together, but only Shimon and Levi – who were perhaps the warriors in the family – carried out the attack on Shechem. On the other hand, it seems difficult to imagine that only two brothers would be sent to attack the city if they all took part in the scheme.Yaakov’s condemnation of the attack – both here and on his deathbed - are reserved for Shimon and Levi. But why does the Torah speak of "Yaakov's sons" without specifying Shimon and Levi?
We read in Parashat Vayishlach the tragic story of Dina’s abduction and defilement by Shekhem, the prince of the city with that same name (chapter 34). After Dina’s abduction, Shekhem and his father approach Yaakov and his sons to propose that the two clans join through marriage and economic cooperation. Yaakov’s sons, speaking with what the Torah described as mirma (literally, “deceit”), express their approval of the plan, on condition that all the males of Shekhem agree to undergo circumcision. Shekhem succeeds in persuading the people of his city to accept these terms, and all the men are circumcised. Three days later, Shimon and Levi assault the city, kill every male, loot the city’s property, and rescue their sister.
Among the questions raised concerning this incident was whether Yaakov’s other sons, besides Shimon and Levi, were party to this scheme. In describing the exchange between Yaakov’s family and Shekhem, the Torah tells that “Yaakov’s sons deceptively answered Shekhem and his father Chamor” and stipulated the condition that the people of Shekhem undergo circumcision (34:13). This would clearly suggest that all of Yaakov’s sons took part in this deception. Later, however, we read that only Shimon and Levi attacked the city (34:25). One might suggest, at first glance, that all the brothers conspired together, but only Shimon and Levi – who were perhaps the warriors in the family – carried out the attack. On the other hand, it seems difficult to imagine that only two brothers would be sent to attack the city if they all took part in the scheme. Wouldn’t they want as many men as possible to undertake this daring mission? Furthermore, Yaakov’s condemnation of the attack – both here and as he lay on his deathbed, as recorded in Parashat Vayechi – are reserved for Shimon and Levi, indicating that only they schemed against the people of Shekhem.
The question thus remains, why does the Torah speak of “Yaakov’s sons,” without specifying Shimon and Levi, as the ones who presented the condition of circumcision?
This question led a number of commentators to suggest that all the brothers conspired against Shekhem – but not with the intent of murdering its male population; this plan was devised only by Shimon and Levi. The Ramban (34:13) explains that the brothers did not expect the people of Shekhem to agree to undergo circumcision. By making Dina’s marriage contingent upon the people’s circumcision, Yaakov’s sons essentially sought to sabotage the deal proposed by Shekhem, so that Dina could be safely returned and not married to Shekhem. And in the unlikely event that the Shekhemites agreed to the condition, the brothers felt, they could easily enter the city while the men recovered from the procedure and rescue Dina without confrontation. It was only Shimon and Levi who, independently, decided to resort to violence. (See also Rav Shimshon Refael Hirsch.)
Shadal explains along similar lines, though in his view, the brothers never even imagined that the people of Shekhem would agree to circumcise; such a possibility never entered their minds. They imposed the condition of circumcision as an excuse for rejecting Shekhem’s offer, so that Dina can be returned. But the Shekhemites’ unexpected consent to this condition put Yaakov’s family in a very difficult position: now that the people of Shekhem had fulfilled their side of the deal, Yaakov’s family had to fulfill theirs, and allow Dina to marry Shekhem. Shimon and Levi refused to allow this to happen, and they therefore took up arms against the city of Shekhem. Rather than abide by their side of the bargain, Shimon and Levi felt that revenge was in order – a stance that their father would later sharply condemn.
(See also Chizkuni, Rabbenu Yosef Bekhor Shor and Abarbanel.)