"Fascism: What it is and how to fight it" is a compilation of essays by Leon Trotsky. It was first published by Pioneer Publishers in August 1944, however, it is not copyrighted. It describe fascism as:
"a spontaneous movement of large masses, with new leaders from the rank and file. It is a plebian movement in origin, directed and financed by big capitalist powers. It issued forth from the petty bourgeoisie, the slum proletariat, and even to a certain extent from the proletarian masses"
It explains how this type of movement can quickly become corrupted, and how in fascism:
"for the most part that the workers' organizations are annihilated; that the proletariat is reduced to an amorphous state; and that a system of administration is created which penetrates deeply into the masses and which serves to frustrate the independent crystallization of the proletariat."
It then outlines how Benito Mussolini rose to power in Italy, and the state of the government of Germany at the time, which was on the verge of adopting a form of fascism itself, under Adolf Hitler and the Nazi Party. He goes on to describe the proletariat and the bourgeoisie classes, and under what conditions they will revolt. Lastly he describes the danger that the United States are in, and, in a way that is relevent in our world today, how to defeat fascism, and the necessity to build what he calls the revolutionary party.
The entire text of the pamphlet can easily be found by searching for "fascism: what it is and how to fight it" in most search engines | 
