From fraternal + -ize.

fraternalize (fraternalizes, present participle fraternalizing; simple past and past participle fraternalized)

1. (transitive) To make fraternal or brotherly; to rid of conflict; to unite.
   • 1999, Josef Škvorecký, Paul Wilson, The Engineer of Human Souls:

     It really bothered him to be one of the first musicians who had to play for the Russians after the entry of the fraternalized armies into Prague.

2. (intransitive) To fraternize; to socialize.