Complex sentence: Elías Calles was president in the period 1924-1928, but in the next six years, Calles was the de facto leader of Mexico, there were three presidents, all of them subordinate to a lesser or greater extent to Calles.
Split into simpler sentences: Elías Calles was president in the period 1924-1928, but throughout the next six years, Calles was the de facto leader of Mexico. There were three presidents in this era, all of them subordinate to a lesser or greater extent to Calles.