Upon the great river, the river Euphrates (epi ton potamon ton megan ton Euphratēn). The sixth trumpet brings up the river Euphrates also (Rev 9:14), only there epi with the locative, while here epi with the accusative. Note triple use of the article ton here.
Was dried up (exēranthē). First aorist (prophetic) passive of xērainō (Rev 14:15). Cf. Zec 10:11.
That may be made ready (hina hetoimasthēi). Purpose clause with hina and the first aorist passive of hetoimazō. Common verb in Rev (Rev 8:6; Rev 9:7, Rev 9:15; Rev 12:6; Rev 19:7; Rev 21:2).
The way for the kings (hē hodos tōn basileōn). Objective genitive basileōn.
That come from the sunrising (tōn apo anatolēs heliou). “Those from the rising of the sun,” the kings from the east (cf. Mat 2:2) in their march against Rome. Parthia in particular resisted Rome before Trajan’s day.