Although the Israelites were divided into Twelve Tribes, the Jews (being one offshoot of the Israelites, another being the Samaritans) are traditionally said to descend mostly from the Israelite tribes of Judah (from where the Jews derive their ethnonym) and Benjamin, and partially from the tribe of Levi, who had together formed the ancient Kingdom of Judah, and the remnants of the northern Kingdom of Israel who migrated to the Kingdom of Judah and assimilated after the 720s BCE, when the Kingdom of Israel was conquered by the Neo-Assyrian Empire.
Who assimilated before the 720s BCE?