What evidense does Howard Zinn provide that "some people in the United States" always fought for justice?

I think
that Zinn's construction of American History is one where he presents how there has been the
development of power as a part of the narrative.  Yet, at the same time, Zinn wants to reveal a
part of this narrative whose dialectic shows people who wanted change, as well.  Zinn believes
that these individuals "always fought for justice" despite the fact that the power
construction tilted against them....