John Calvin Commentary Deuteronomy 33:22

"And of Dan he said, Dan is a lion`s whelp, That leapeth forth from Bashan." — Deuteronomy 33:22 (ASV)

And of Dan he said. He foretells that the tribe of Dan, like that of Gad, should be warlike, not so much from voluntary disposition as from necessity; for their love of war was not to be deemed praiseworthy, since it is altogether contrary to humanity; but because the unscrupulousness of the enemies that infested that tribe compelled them to take up arms.

He compares it to a lion impetuously leaping from Mount Bashan; and the comparison must be understood here, for Mount Bashan was not situated in the territory of this tribe. But Moses means to say that they should be as ready for combat as a lion, which, after it has emerged from its den into the open plains, attacks everyone it meets.