Jamieson, Fausset and Brown on Jeremiah 50:21

21. Merathaim--a symbolical name for Babylon, the doubly rebellious, namely, against God. Compare Jr 50:24, "thou hast striven against the Lord"; and Jr 50:29, "proud against the Lord." The "doubly" refers to: first, the Assyrian's oppression of Israel; next, the kindred Chaldean's oppression of Judah (compare Jr 50:17-20, 33; especially Jr 50:18).

Pekod--(Eze 23:23); a chief province of Assyria, in which Nineveh, now overthrown, once lay. But, as in Merathaim, the allusion is to the meaning of Pekod, namely, "visitation"; the inhabitants whose time of deserved visitation in punishment is come; not, however, without reference to the now Babylonian province, Pekod. The visitation on Babylon was a following up of that on Assyria.

after them--even their posterity, and all that is still left of Babylon, until the very name is extinct [Grotius]. Devastate the city, after its inhabitants have deserted it.

all ... I ... commanded--by Isaiah (Is 13:1, &c.).