Nehemiah 2:7

Moreover I said unto the king, If it please the king, let letters be given me to the governors beyond the river, that they may convey me over till I come into Judah;

Let letters be given me to the governors beyond the river. The Persian empire at this time was of vast extent, reaching from the Indus to the Mediterranean, and the Euphrates was considered as naturally dividing it into two parts, eastern and western (see Ezra 5:3).