Sentence:  Constantius was not born to the purple and Maximianus was the only original member of the First <WSD>Tetrarchy</WSD> from whom Constantine could satisfactorily derive his rule. 
Meaning of the word marked with <WSD> tags:  A government where power is shared by four people, "especially"  the Herodian tetrarchy established in Judea after the death of Herod and the Tetrarchy of Diocletian which ruled the Roman Empire in the years 293-313.