A.T. Robertson Commentary Revelation 16:12

"And the sixth poured out his bowl upon the great river, the [river] Euphrates; and the water thereof was dried up, that the way might by made ready for the kings that [come] from the sunrising." — Revelation 16:12 (ASV)

Upon the great river, the river Euphrates (επ τον ποταμον τον μεγαν τον Ευφρατην). The sixth trumpet brings up the river Euphrates also (9:14), only there επ with the locative, while here επ with the accusative. Note triple use of the article τον here.

Was dried up (εξηρανθη). First aorist (prophetic) passive of ξηραινω (14:15). Cf. Zechariah 10:11.

That may be made ready (ινα ετοιμασθη). Purpose clause with ινα and the first aorist passive of ετοιμαζω. Common verb in Rev. (8:6; 9:7,15; 12:6; 19:7; 21:2).

The way for the kings (η οδος των βασιλεων). Objective genitive βασιλεων.

That come from the sunrising (των απο ανατολης ελιου). "Those from the rising of the sun," the kings from the east (cf. Matthew 2:2) in their march against Rome. Parthia in particular resisted Rome before Trajan's day.