Marxists knew what fascism was is the interwar period but it seems you have lost touch. If I wanted to know what Marxism was I'd find out from a Marxist not a liberal. Now like liberals it's common for communists to call anything they don't like fascist. That bothers me because I don't appreciate being likened to those that consort with or excuse Sodomites.
Fascism is the belief that society is to be organised by natural law in order to create an organic state. That organic state will manifest differently depending on culture and religion of a people. Organic states are inevitable. Peoples once lived in organic states during the feudal era but all those institutions have since been destroyed which leaves us now the revolutionary option. Fascists are revolutionaries not reformists and fascists are spiritualists not materialists.
On the question of authoritarianism which liberals point to as the defining characteristic of fascism. It is and it isn't since you can't create an organic state over night. It's not a belief in authoritarianism that makes a fascist but rather that authoritarianism is a necessity to make the transition to an organic state. All the diseases that infect nations will have to be purged.
The organic state is inevitable since we are at the end stages of a dying civilisation that deserves its ending and cannot be saved. Fascists could do nothing and eventually organic states will form after centuries of suffering and hardship. Fascists want to spare people all of the suffering that is coming. We want good people to live well, fruitful and comfortable so those good people can create more good people like themselves while we cut out the diseases that would bring the good people down with it. It's like when you have a diseased crop and you uproot the diseased crops to stop the disease from spreading to the healthy crops. The only alternative is to let the whole crop succumb to disease. A disease doesn't know it's a disease. It only wants to spread and doesn't know that it is killing its host.