Albert Barnes Commentary 2 Samuel 17:12

"So shall we come upon him in some place where he shall be found, and we will light upon him as the dew falleth on the ground; and of him and of all the men that are with him we will not leave so much as one." — 2 Samuel 17:12 (ASV)

As the dew — Like the drops of dew, this refers to the vast number of our army and our irresistible and unavoidable descent upon our enemies.