Constantine’s role was as a politician as a peacemaker trying to get two quarreling religious factions together. A thousand years later, Charles V tried through a serious of meetings to get the Lutherans and the Papists together in Speyer and other places. Unable to get them to agree, it all ended in a civil war. Constantine had just ended such a war and hoped by sponsoring the Christians that he could bring peace to the empire.
The historical record shows: the term "homoousios" was first condemned at the Synods of Antioch (264 - 268 AD) and then inserted by Constantine himself (according to Eusebius of Caesarea) at his Council of Nicaea in 325 AD.
Does that make it, in your word, a "diktat"?
No, because so far as we know, a majority at Nicaea were perfectly willing to accept "homoousios", despite it's previous condemnation.
But I'd be more inclined to think the majority then, as now, was not very energized by fine points of distinctions in technical terms that nobody really understands anyway.
So they were happy to vote with the stronger side, especially since that was supported by their Emperor.
And besides, Arius himself was a bit of a thorn, a mere presbyter, who had required previous disciplinary actions.
Remember, Arius started with support from just 22 bishops -- around 10% of attendees -- and all but two eventually changed sides to sign the homoousians' Creed.
RobbyS: "...Constantine’s diktat. But their judgment went against his views.
He kept trying to rehabilitate Arius ,
This came close to happening and his successor Constantius was an Arian."
At the time of Nicaea, Constantine first inserted "homoousios" into the Creed, then condemned to death anyone caught with copies of Arius' writings.
So Constantine's views at that time were strong and unequivocal.
Later, Constantine was persuaded that Arian ideas made some sense, and appeared willing to give them another hearing.
Some of Constantine's successors supported Arian ideas, and Arianism was not completely eradicated for many generations.
Indeed, you might even argue that theologically, Islam is simply Arianism revisited.
RobbyS: "As to both sides, Arius had his hearing and a majority of the court decided against him on a theological issue, one involving the interpretation of the Bible."
A politically motivated "court", unduly influenced by the Emperor's secular power, whose edict carried the death penalty in a matter that should have been left to individuals' interpretations (within biblical limits) as the spirit moved them.
In short: there should have been room for both views, and likely would eventually, except for the Emperor's influence.
RobbyS: "Constantine had just ended such a war and hoped by sponsoring the Christians that he could bring peace to the empire."
Christians warring against other Christians over theological issues: never seen before the age of Constantine.
Indeed, Constantine's first concern was to sharply distinguish those he supported from those he intended to physically destroy.
That is Constantine's woeful legacy.