Complex sentence: Until the mid- 20th century, there were Jews in some parts of the Levant; now most are in Israel, joining those who returned when the Modern State of Israel was established in 1948.
Split into simpler sentences: Until the mid- 20th century, there were Jews in some parts of the Levant. Most are now in Israel, joining those who immigrated to the State of Israel, when it was established in 1948.