This Bible contradiction is from the Skeptic's Annotated Bible.

Verses

Do not fight against them or take their land.

Deuteronomy 2:19 View context

And [when] thou comest nigh over against the children of Ammon, distress them not, nor meddle with them: for I will not give thee of the land of the children of Ammon [any] possession; because I have given it unto the children of Lot [for] a possession.

Kill them and take their land.

Judges 11:32 View context

So Jephthah passed over unto the children of Ammon to fight against them; and the LORD delivered them into his hands.

Jeremiah 49:2 View context

Therefore, behold, the days come, saith the LORD, that I will cause an alarm of war to be heard in Rabbah of the Ammonites; and it shall be a desolate heap, and her daughters shall be burned with fire: then shall Israel be heir unto them that were his heirs, saith the LORD.