Parashas Vayechi · Passuk 49:8
Yehudah, thee shall thy brethren praise; thy hand shall be on the neck of thine enemies; thy father’s sons shall bow down before thee.
Torah Temimah points to Megillah 16a.
English: R’ Yochanan said: What is the meaning of that which is written “Then said his wise men and Zeresh his wife unto him, If Mordechai be of the seed of the Jews \ldots thou shalt not prevail against him”? They said to him: If he comes from the other tribes you can prevail against him; but if from the tribe of Yehudah or Binyamin or Efrayim or Menasheh you cannot prevail against him. Yehudah — as it is written “thy hand shall be on the neck of thine enemies.”
Derash linking the blessing to later history — the verse is the source of the invincibility attributed to Yehudah.
The blessing “thy hand shall be on the neck of thine enemies” is the reason that one descended from Yehudah cannot be overcome by ordinary means.