Complex sentence: Trotsky, Kamenev and Zinoviev were ejected from the Central Committee and then expelled from the Party, and while Kamenev and Zinoviev were readmitted, but Trotsky was exiled from the Soviet Union.
Split into simpler sentences: Trotsky, Kamenev and Zinoviev were ejected from the Central Committee and then expelled from the Party. While Kamenev and Zinoviev were readmitted, Trotsky was exiled from the Soviet Union.