What does Genesis 34:15 mean?

Jacob's sons remain outraged at the violation of their sister, Dinah, at the hands of Shechem (Genesis 34:1–7). They have been offered the ability to name their own price to allow Dinah to marry her attacker (Genesis 34:8–12). It's not hard to imagine that Dinah's brothers were more interested in revenge than in negotiations. However, their options are limited. Dinah remains with Shechem's people (Genesis 34:26). Though Jacob's family is wealthy (Genesis 30:43), they would not have numbered the same as an entire city (Genesis 33:18).

Rather than beginning an immediate assault, the men devise a scheme (Genesis 34:13). They pretend to agree with Shechem's proposal, given one condition: every male in Shechem must be circumcised as Jacob and his sons had been. This practice of circumcision was a requirement given by God to Abraham for him and all his male descendants. It was a condition of His covenant with His people. Jacob and his sons would have been circumcised as infants. Circumcision was clearly not practiced by the men of Shechem.

More important to Jacob's sons, of course, is that circumcising an adult male—removing the foreskin of the penis—makes that man temporarily unfit for combat.