Which Council of Nicea? There is a lot of information available online and at the library concerning the general and ecumenical councils of the early church. I believe the 1st Council of Nicea is the 1st ecumenical council in any list of Catholic and Orthodox Church councils.
This council was called by Constantine to settle disagreements between Christian factions. From what I have read these factions hotly contested each other and fought for supremacy. You need to read more than one source since the Catholic Church puts one spin on this council and anti-Catholics put another spin on it. For instance the Catholic Church claims Pope Sylvester called this council and the Orthodox Church denies this, while seculare histories side with the Orthodox Church since Sylvester did not attend the council and the preserved council minutes plainly state Constantine convened the council and the Bishop of Constantinope (or Antioch) presided.