Alexander’s letter to his namesake in Byzantium is a long and repetitive statement of the Arian problem. His Catholic Epistle is a more succinct setup for Nicea.
He tries to stop certain readings of the Christian books but his opponents are slippery -
by saying these things, and by unfolding the divine Scriptures, we have often refuted them. But they, chameleon-like …. Their arguments still resonate with many. Alexander formally bans,
anathematized such men, but that fails to stop other powerful bishops supporting them including Eusebius of Nicomedia.
The stage is set for the novelties of the Council. A precise definition of the orthodox position and its banned alternative to take out the fence sitters. Anathema again but now enforced Rome-wide to remove or neuter powerful bishops like Eusebius.
You can argue that the creed is valuable - God is made clear even if his books are ambiguous - but state-intervention? Slippery, slippery … To enforce the position of its majority, the Church needed Constantine. They made him a protagonist in their drama and he assumed the role offered. In spite of the Constantine-bashers, there was no machiavellian at Nicea. Just a majority-pleaser, a God-pleaser.