The tool the fascists have used the most to get their way is the military (i.e. all the fascist coups). At the end of the day the people that have the power are the ones with the guns. Being in the military attracts fascists more so than any other political ideology because:
A. they are heavily nationalistic
B. with being in the military it lets them legally kill and beat up people
Now I know I may get some push back from this, especially from those of us in the United States (which I am too). The push back I'm gonna get is that the US military is a tool of big business, which it is. However, we have to start somewhere and we definitely CAN NOT just let it be overtaken by fascists. Having leftists in the millitary to oppose the fascism in it is a much more powerful weapon than leftists that form a rebel/guerilla group outside the millitary. A guerilla group in the United States has a near zero chance of being successful, while those in the military who are leftists can use their power and the people's trust to oppose fascism on its own familiar turf. Look at the history of Latin America for instance. Left-wing military coups to force out fascist military leaders have been much more successful than left-wing guerrilla groups. Some left wing military coups in Latin America have been from: Juan Jose Torres (Bolivia), Manuel Isidoro Belzu (Bolivia), Velasco (Peru), David Toro (Bolivian). If you haven't noticed by now I am half Bolivian hence why I'm more likely to know a left-wing general if he's from Bolivia. And of course had it not been for Hugo Chavez purging the Venezuelan military Venezuela would still be under the control of international companies.
And the only reason Pinochet's first coup attempt failed was because it was put down by the members of the military who were loyal to the Chilean constitution.