Albert Barnes Commentary Jeremiah 31:2

"Thus saith Jehovah, The people that were left of the sword found favor in the wilderness; even Israel, when I went to cause him to rest." — Jeremiah 31:2 (ASV)

The people who were left of the sword - A promise of the restoration of the ten tribes to their land.

The wilderness - Either the desert that lay between Assyria and Palestine, or more probably an allusion to the wilderness of Mount Sinai.

Found grace ... rest - Rather, shall certainly find grace; I will go to give Israel rest.