Albert Barnes Commentary Judges 2:3

"Wherefore I also said, I will not drive them out from before you; but they shall be [as thorns] in your sides, and their gods shall be a snare unto you." — Judges 2:3 (ASV)

Wherefore I also said—Rather, because you have done the things mentioned in (Judges 2:2), “I have now said (that is, I now protest and declare) that I will not drive them out from before you” . And it was the announcement of this resolution by the Angel that caused the people to weep.

The word 'thorns' in this verse is supplied by the King James Version from the similar passage in Joshua (see the marginal reference). Other versions adopt a different reading of the original text, and prefer the sense they shall be to you for adversaries (Compare to the last words of Numbers 33:55).