Albert Barnes Commentary Jeremiah 50:24

"I have laid a snare for thee, and thou art also taken, O Babylon, and thou wast not aware: thou art found, and also caught, because thou hast striven against Jehovah." — Jeremiah 50:24 (ASV)

I have laid a snare for thee - Babylon, the impregnable, was taken (according to Herodotus) by Cyrus by stratagem. Having diverted the waters of the Euphrates, he entered the city by the river channel: but see Daniel 5:1 note.

And thou wast not aware - Better literally, and you did not know it.