Complex sentence: During most of the twentieth century other re-enactments could not be held because of the establishment of the USSR after the 1917 Russian Revolution until glasnost in the early 1990s racers were again allowed on this race.
Split into simpler sentences: During most of the twentieth century other re-enactments could not be held, because of the establishment of the USSR after the 1917 Russian Revolution. After glasnost in the early 1990s, racers were again allowed on this race.