The council was held in September 381 AD and summoned by Gratian, the Western Roman Emperor, explicitly to "solve the contradictions of discordant teaching" was in fact organized by Ambrose, though it was presided over by Valerian, Bishop of Aquileia.
Politics and Christology were inextricably entangled in the 4th century: "You have contrived, as appears by the sacred document (Gratian's amended convocation) which you have brought forward, that this should not be a full and General Council: in the absence of our Colleagues we cannot answer", was Palladius' stand.
Ambrose proposed that Arius' letter from Nicomedia to Alexander, bishop of Alexandria, should be read in detail, and Palladius be called upon to defend or condemn each heretical proposition that disputed Catholic orthodoxy.
Ambrose rested upon the verbal formulas recently agreed upon by authority of the Church, while Palladius refused to admit the legitimacy of the proceedings.
Scolies Ariennes sur le Concile d'Aquilee, introduction, text, and notes Roger Gryson, Sources chretiennes 267 (Paris: Cerf, 1980).