Complex sentence: Trotsky's resignation was unanimously accepted by the Central Committee at the January 18 session, which he did not attend; efforts by Zinoviev and Kamenev to remove Trotsky from the Politburo and to expel him from the party were turned aside, however.
Split into simpler sentences: Trotsky's resignation was unanimously accepted by the Central Committee at the January 18 session, which he did not attend. Efforts there by Zinoviev and Kamenev to remove Trotsky from the Politburo and to expel him from the party were turned aside, however.