The people who lived there in Edom were descendants of Esau, whereas Israel and Judah were descendants of Jacob (Genesis 25:25–26). Edom and Israel/Judah were frequently at war with one another (2 Chronicles 28:17), so much so that Edom receives special condemnation in Obadiah, the book that follows Amos.
11 Thus says the LORD: “For three transgressions of Edom, and for four, I will not revoke the punishment, because he pursued his brother with the sword and cast off all pity, and his anger tore perpetually, and he kept his wrath forever.