Fascism was a political philosophy originating on Earth. After Adolf Hitler took power in Germany in the 1930s and established the Nazi state it became his government style. (TOS: "The City on the Edge of Forever").
A key pillar of fascism was the so-called "Leader Principle" which stated that a government should have a single leader as its head, with all others obeying this leader without question. In the 20th century fascist leaders were generally deemed evil and psychotic men and it was for this reason that fascism failed on Earth as a viable form of government. However, even in cases where a fascist leader governed for the good of the people, it was a fact that when any man held so much power then it was in turn irresistible that an urge to "play God" would corrupt the government. Such was the case on Ekos, where Federation historian John Gill founded a peaceful fascist government only to have his government turn evil just like its predecessors. (TOS: "Patterns of Force")