Judges 3:13

And he gathered unto him the children of Ammon and Amalek, and went and smote Israel, and possessed the city of palm trees.

Cross-reference

Judges 6:3 also features Amalekites as part of an invading coalition, showing a recurring pattern of Amalek joining enemies.

Psalm 83:7 lists Ammon and Amalek together, exactly the two groups Eglon gathered — a direct parallel of the coalition.