Complex sentence: During a substantial migration that took place in the 17th century, these Jewish refugees from the Iberian peninsula called themselves Portuguese Jews to avoid being identified with Spain, which was at war with the Dutch Republic at the time (see Eighty Years' War).
Split into simpler sentences: During a substantial migration that took place in the 17th century, these Jewish refugees from the Iberian peninsula called themselves Portuguese Jews. They wanted to avoid being identified with Spain, which was at war with the Dutch Republic at the time (see Eighty Years' War).