As you know i live in Serbia, this is first time i hear for this.
The Serbia had ties with Russia where Putin use the Night Wolves mc as his muscle on the streets. The Wolves are homophobic and politically deployed (his founder gained a crime medal from putin) and the angels in russian charters are against the wolves so this gathering in Serbia maybe is a warning to the wolves?
Night Wolves are said to be Putin's friend(he said it himself, he was at one of their rallies). It is said that they were involved in the Crimea return to Russia, members involved in military acts. The leader received not a crime medal, but Medal "For the Return of Crimea".
That's a possibility what you state, but I don't see how they can warn them of being in Serbia. Hells Angels got a lot of chapters all around Europe. Even in Latvia(the MC Bears were turned into HA), which is a border country with Russia. They don't have any support from the government as of what we know as in most countreis they are considered as an illegal gang. But Night Wolves got support of the Russian government, which means they can easily take out any competition within Russia if they want it. Even Chechnya leader Ramzam Kadyrov is a member of them.
I bet if Hells Angels pulls anything on Night Wolves, next thing you'll see is them die off Novichok being sprayed all over them...
