Miley's New Boyfriend -- History of Violence

Just a few months ago -- before he and Miley hooked up -- 19-year-old Liam (in the gray striped shirt) and his brother ("Star Trek" actor Chris Hemsworth) got into a full-on fistfight with some random dude after an event in Hollywood.

We're told it all went down after the brothers -- who are currently featured in Vanity Fair -- engaged in some late-night smack talk with the random guy. The brothers approached the dude -- and suddenly a 2-on-1 brawl broke out.

We're told event security broke up the fight --and managed to separate everyone before cops were needed.