Complex sentence: During the Russian Civil War, when Aleksandr Kolchak ruled in Siberia, he introduced around 1918 a winter uniform hat, commonly referred to as a "kolchakovka", which was similar to the ushanka but had an extra eye-flap.
Split into simpler sentences: During the Russian Civil War, the ruler of Siberia, Aleksandr Kolchak, introduced a winter uniform hat, commonly referred to as a "kolchakovka", c.1918. It was similar to the ushanka but had an extra eye-flap.