It's easy to succumb to the nihilism in the face of fascism because it is an ideology that is unencumbered by truth or meaning. People like to think that if they could just have a conversation with a fascist and reason with them that they could convince them otherwise. When all the reason in the world does not stem the tide of fascism, one becomes disillusioned with the ideals of the enlightenment. Fascism is the antithesis of the enlightenment: fascism is the full embrace of the stupidity of violence and domination. It is rooted in that fundamental basis of social order which few examine critically. We all would like to believe that our civilization has gone far beyond the point where it is brute violence that decides social truth, and when fascism rises it lifts with it the veil that allows us to entertain such fantasies. When all of the things we were taught are the basis of society seem to be completely worthless in the face of fascism's brutality, how could we but not feel helpless and passive?

Here's an excerpt from Sartre's Anti-semite and Jew [1]. I think it's illuminating:

  The anti‐Semite has chosen hate because hate is a faith; at    the outset he has chosen to devaluate words and reasons.     How  entirely  at  ease  he  feels  as  a  result.   How  futile  and    frivolous discussions about the rights of the Jew appear to    him.   He  has  placed  himself  on  other  ground  from  the    beginning.   If out of courtesy he consents  for a moment to    defend his point of view, he lends himself but does not give    himself.   He  tries  simply  to  project  his  intuitive  certainty    onto the plane of discourse.  I mentioned awhile back some    remarks  by  anti‐Semites,  all  of  them  absurd:  "I  hate  Jews    because  they  make  servants  insubordinate,  because  a    Jewish  furrier  robbed  me,  etc."   Never  believe  that  anti‐   Semites  are  completely  unaware  of  the  absurdity  of  their    replies.  They know  that  their remarks are  frivolous, open    to  challenge.   But  they  are  amusing  themselves,  for  it  is    their  adversary  who  is  obliged  to  use  words  responsibly,    since he believes in words.  The anti‐Semites have the right    to play.  They even like to play with discourse for, by giving    ridiculous  reasons,  they  discredit  the  seriousness  of  their    interlocutors.  They delight in acting in bad faith, since they    seek not to persuade by sound argument but to intimidate    and  disconcert.   If  you  press  them  too  closely,  they  will    abruptly  fall  silent,  loftily  indicating  by  some  phrase  that    the time for argument is past.  It is not that they are afraid    of being convinced.  They fear only to appear ridiculous or    to prejudice by their embarrassment their hope of winning    over some third person to their side. 

In order to resist fascism one has to have the conviction to fight for something that fascism seeks to destroy.

[1] https://www.amazon.com/Anti-Semite-Jew-Exploration-Etiology-...