Complex sentence: In the post-World War II period, from 1948 to the early 1950s about 50,000 Belarusians immigrated to the United States; most of them left Europe for political reasons.
Split into simpler sentences: In the post-World War II period, from 1948 to the early 1950s, many Belarusians arrived in the United States. During this period about 50,000 Belarusians immigrated to the United States, most of them having left Europe for political reasons.