Complex sentence: In 1654 after Cossack-Russian Treaty of Pereyaslav he switched sides and allied with Poland against Russia; died soon afterwards assassinated by his Cossack concubine.
Split into simpler sentences: In 1654 after the Treaty of Pereyaslav, he switched sides and allied with Poland against Russia. He died soon afterwards when he was assassinated by his Cossack concubine.