If Antifa in the US is the similar to the ones in Sweden they will be initiating violence at protests like this. They are usually very prepared and have a lot of experience how to do it. Using violence to fight fascism is what they do even though they usually use non lethal methods.

In Sweden they consist of small autonomous groups that harass nazi sympathizer by vandalizing their homes, putting their address on the internet and calling relatives, neighbors and their work etc. Sometimes they have also physically assaulted nazis. In protests they usually throw stones, pepper spray, smoke bombs, urine etc and sometimes try and brawl with their opponents using clubs etc.

But I don't know at all what happened at this rally in the US but I would be incredibly surprised if you had one of the biggest white supremacy rally is in a long time in the US and Antifa did NOT try and physically stop it.