the soldiers = some soldiers (no Art.) going on service. Not the Noun, but the Participle = men under arms. Josephus (Antiquities xviii 5 § 1,2) tells us that Herod Antipas (Luk_3:1) was engagedin a war with Aretas his father-in-law, a petty king in Arabia Petrea, at this very time, and his soldiers were passing from Galilee through the very country where John was proclaiming.
Do violence = terrify with a view to extortion. Occurs only here in the N.T.