Complex sentence: Rodzianko supported Anton Denikin and Pyotr Wrangel but when it became clear the White Army had lost he emigrated to Serbia in 1920, wrote his memoirs "The Reign of Rasputin".
Split into simpler sentences: Rodzianko supported Anton Denikin and Pyotr Wrangel but when it became clear the White Army had lost he emigrated to Serbia in 1920. There he wrote his memoirs "The Reign of Rasputin".