Plato
Plato was not the very first fascist, but he was the first to put fascist ideals to paper (that we know of). Fascism is not new. It is quite ancient. Mussolini may have coined the term, but the core concepts of fascism are all there in Greek Philosophy. Which was an ideal state meritocracy incorporating nationalism and justice.
Beliefs
For Plato and Socrates, everything is about producing an objective function. Humans must perform an objective role. And that role is individual responsibility. Aesthetic things like entertainment and music must also conform to some objective function. Otherwise in Plato's eyes it has no reason to exist. And that objective is to promote nationalism.
The perfect man (Neitchze's uberman) according to Plato was a soldier. Who would fight and die for his country. This even today is the ultimate exptession of patriotism. The only time the truth is not allowed to exist. Is when that truth contradicts nationalism and the law.
He believed that the highest crimes, worthy of a death penalty should be reserved for corrupt officials. If there is some truth that society's criminals live long and happy lives, such a horrible thing should be changed.
State secrets are a necessary evil, but everything else must be true. Art has to be objectively true. Music has to be objectively true. Meaning that paintings and lyrics have to objectively portray what people looked like and behaved like based on the facts available. Which is why Plato was opposed to painting sculptures with purple eyes...because no Greek as purple eyes. (His example) Doing such a thing is no different than lying.
He believed that the upper class should not be allowed to own private property, or at least very little, and should make a vow to use their wealth for the public good. This is a recurring theme am9ng modern fascists. He also believed in eugenics. He thought the upper class could be selectively bred to produce the best genes. So they can be the most perfect soldiers, artists, and leaders.
Plato's Republic
His ideal Republic was hardly a democracy. It was fascism, and if his ideas were transposed to modern times you'd have a state run by a technocratic-philosopher elite. This compares to modern fascists over the last 100 years.
The idea behind the Republic is fitted with what many Greeks would have believed in; that working people would have no time to consider fully the debates and issues of the day. This still applies to this day.
How was Plato a fascist?
The idea that Plato was fascist is absolutely correct. Just because the ideology did not exist as a term at the time doesn’t mean it didn’t exist in the mindset.
Plato's ideas followed very closely to the ideology. Some tefer to h as a "proto-fascist", but that term teally belongs to Lycurgus, the Spartans, and Socrates.
Plato, as stated in "The Republic" (Plato's original word for his fascist system), believed in an authoritarian state that instilled culturally enriched learning, a strong military, unified people that were against the blight of riots, and eugenics (selected breeding and a hallmark of fascism) in order to further the progress and quality of human kind, also mentioning how his “guardians” would be an elite class of people who were bred to have superior genes and be head of the state, through meritocracy. These beliefs were the very first fascist beliefs: an authoritarian, nationalist government that has a strong military power and restrictions on rioting and other destructive activity.
So to make it short, yes Plato was a fascist. Saying otherwise is childish and ignorant to the fact you wish not to assosciate the man with an ideology (the truth being that you just don't like the idea of a figure like Plato being a fascist) all because “the word didn’t exist then”.
Law of Decay
Plato delineates “the law of decay” that he believed underpinned all human societies: a temporarily ideal society inevitably first morphs into a timocracy, where personal honor forms virtuous society; the timocracy which will then result in democracy, where all pursuits are honored equally and the state is at the mercy of relentless tribal conflict. This devenerates into oligopoly, where the avarice of an enriched minority will rule the day; this fiduciary perversion and strife will eventually give rise to authoritarianism or possibly even tyranny, where a strong leader will correct the society, hopefully with love, but possibly ruling without temperance or virtue. To escape this degenerate cycle, Plato argued that society needed the guidance of philosopher-kings, whose biology and education would uniquely equip him to maintain the true Form of the Kallipolis fascism, and stave off the otherwise inevitable societal decay.