Complex sentence: Early in the 16th century Henry VIII imported a large number of stallions and mares for breeding although it was not until the 17th and 18th centuries that the breeding of Thoroughbreds began as we know it now.
Split into simpler sentences: Early in the 16th century Henry VIII imported a large number of stallions and mares for breeding. It was not until the 17th and 18th centuries, however, that the breeding of Thoroughbreds began as we know it now.