Complex sentence: His greatest coup was the capture of Perm, where about 20,000 Red Army soldiers were taken prisoner on 24 December 1918, but further advance on Vyatka was impeded by heavy frost.
Split into simpler sentences: His greatest coup was the capture of Perm, where about 20,000 Red Army soldiers were taken prisoner on 24 December 1918. Further advance on Vyatka was impeded by heavy frost.