unneutered toms can be hostile in a very noisy way.
Normally the two approach each other with tails moving slowly from side to side, all the while making direct eye contact. The smaller of the two cats may size up the situation and slink away, but if two cats feel they are evenly matched, they will walk past each other; then, one will spring onto the other, who will roll onto his back.
The one on top will try to bury his teeth into the other’s nape. While lying belly to belly this way, they will claw, bite, urinate on each other and create the noisiest ruckus cats are capable of. At some point the attacker will jump free, giving the other a chance to slink away or engage in a counterattack.