Jamieson, Fausset and Brown on Jeremiah 2:18

18. now--used in a reasoning sense, not of time.

the way of Egypt--What hast thou to do with the way, that is, with going down to Egypt; or what ... with going to Assyria?

drink ... waters--that is, to seek reinvigorating aid from them; so Jr 2:13, 36; compare "waters," meaning numerous forces (Is 8:7).

Sihor--that is, the black river, in Greek, Melas ("black"), the Nile: so called from the black deposit or soil it leaves after the inundation (Is 23:3). The Septuagint identifies it with Gihon, one of the rivers of Paradise.

the river--Euphrates, called by pre-eminence, the river; figurative for the Assyrian power. In 625 B.C., the seventeenth year of Josiah, and the fourth of Jeremiah's office, the kingdom of Assyria fell before Babylon, therefore Assyria is here put for Babylon its successor: so in 2Ki 23:29; La 5:6. There was doubtless a league between Judea and Assyria (that is, Babylon), which caused Josiah to march against Pharaoh-necho of Egypt when that king went against Babylon: the evil consequences of this league are foretold in this verse and Jr 2:36.