Complex sentence: The Warsaw Pact invasion of Czechoslovakia in 1968 was followed by a wave of emigration, unseen before; it stopped shortly after (estimate: 70,000 immediately, 300,000 in total).
Split into simpler sentences: The Warsaw Pact invasion of Czechoslovakia in 1968 was followed by a wave of emigration, unseen before. It stopped shortly after (estimate: 70,000 immediately, 300,000 in total).