Albert Barnes Commentary Jeremiah 2:10

"For pass over to the isles of Kittim, and see; and send unto Kedar, and consider diligently; and see if there hath been such a thing." — Jeremiah 2:10 (ASV)

Kedar signifies the whole East, and the isles of Chittim (Isaiah 23:12, see note) the West. If then you traverse all lands from west to east, it will be impossible to find any nation guilty of such apostasy as that committed by Israel.