Translated by Cardinal Newman.
Introduction to THE COUNCILS.
The de Synodis is the last of the great and important group of writings of the third exile. With the exception of S:S:30, 31, which were inserted at a later recension after the death of Constantius (cf. Hist. Ar. 32 end), the work was all written in 359, the year of the 'dated' creed (S:4 apo tes nun hupateias) and of the fateful assemblies of Rimini and Seleucia. It was written moreover after the latter council had broken up (Oct. 1), but before the news had reached Athanasius of the Emperor's chilling reception of the Ariminian deputies, and of the protest of the bishops against their long detention at that place. The documents connected with the last named episode reached him only in time for his postscript (S:55). Still less had he heard of the melancholy surrender of the deputies of Ariminum at Nike on Oct. 10, or of the final catastrophe (cf. the allusion in the inserted S:30, also Prolegg. ch. ii. S:8 (2) fin.).
The first part only (see Table infra) of the letter is devoted to the history  of the twin councils. Athanasius is probably mistaken in ascribing the movement for a great council to the Acacian or Homoean anxiety to eclipse and finally set aside the Council of Nicaea. The Semi-Arians, who were ill at ease and anxious to dissociate themselves from the growing danger of Anomoeanism, and who at this time had the ear of Constantius, were the persons who desired a doctrinal settlement. It was the last effort of Eastern 'Conservatism' (yet see Gwatkin, Studies, p. 163) to formulate a position which without admitting the obnoxious homoousion should yet condemn Arianism, conciliate the West, and restore peace to the Christian world. The failure of the attempt, gloomy and ignominious as it was, was yet the beginning of the end, the necessary precursor of the downfall of Arianism as a power within the Church. The cause of this failure is to be found in the intrigues of the Homoeans, Valens in the West, Eudoxius and Acacius in the East. Nicaea was chosen by Constantius for the venue of the great Synod. But Basil, then in high favour, suggested Nicomedia, and thither the bishops were summoned. Before they could meet, the city was destroyed by an earthquake, and the venue was changed to Nicaea again. Now the Homoeans saw their opportunity. Their one chance of escaping disaster was in the principle 'divide et impera.' The Council was divided into two: the Westerns were to meet at Ariminum, the Easterns at Seleucia in Cilicia, a place with nothing to recommend it excepting the presence of a strong military force. Hence also the conference of Homoean and Semi-Arian bishops at Sirmium, who drew up in the presence of Constantius, on Whitsun-Eve, the famous 'dated' or 'third Sirmian' Creed. Its wording (homoiou kata panta) shows the predominant influence of the Semi-Arians, in spite of the efforts of Valens to get rid of the test words, upon which the Emperor insisted. Basil moreover issued a separate memorandum to explain the sense in which he signed the creed, emphasising the absolute likeness of the Son to the Father (Bright, Introd., lxxxiii., Gwatkin, pp. 168 sq.), and accepting the Nicene doctrine in everything but the name.
 He undertakes to tell haper he& 240;raka kai egnon akribos, words which have given rise to the romantic but ill-founded tradition that, ubiquitious and untiring in his exile, he was a secret spectator of the proceedings of his enemies at these distant gatherings. (So Gibbon and, as far as Seleucia is concerned, Tillemont. Montfaucon, as usual, takes the more sober and likely view.)
Reference address : https://www.elpenor.org/athanasius/councils.asp