Adam Clarke on Jeremiah 2:18

What hast thou to do in the way of Egypt - Why dost thou make alliances with Egypt?

To drink the waters of Sihor? - This means the Nile. See on Isa 23:3 (note).

The way of Assyria - Why make alliances with the Assyrians? All such connections will only expedite thy ruin.

To drink the waters of the river? - The Euphrates, as נהר nahar or הנהר hannahar always means Euphrates, the country between the Tigris and Euphrates, is termed to this day Maher alnahar, "the country beyond the river," i.e., Mesopotamia.

Instead of cleaving to the Lord, they joined affinity and made alliances with those two nations, who were ever jealous of them, and sought their ruin. Egypt was to them a broken reed instead of a staff; Assyria was a leaky cistern, from which they could derive no help.