Albert Barnes Commentary Ezra 4:1

"Now when the adversaries of Judah and Benjamin heard that the children of the captivity were building a temple unto Jehovah, the God of Israel;" — Ezra 4:1 (ASV)

Adversaries - that is, the Samaritans, a mixed race, partly Israelite but chiefly foreign, which had replaced to some extent the ancient inhabitants after they were carried into captivity by Sargon (see 2 Kings 17:6 note).