Robert won the fight with Jake and Mike because he had been a boxer when he was a student at Princeton. Plus, Jake and Mike were drunk. Robert easily knocked them both out, despite Mike saying he was not really knocked out. Robert may have won the physical fight, but he did not win any moral victory. He was in love with Brett and had spent some time with her at San Sebastian. He did not understand Brett and thought that because she had slept with him, it meant something. It meant nothing to Brett.
Robert cannot accept this. He goes to Pamplona for the bullfights and follows Brett wherever she goes, even though she is now with Mike, the man she is supposed to marry. Robert is very serious - Brett tells Jake she "hates his suffering." Robert thinks he has lost Brett, but he really never had her to begin with. He has been a kind of loser throughout the novel. No one really likes him, they just put up with him. He is Jewish and the others seem to be very anti-semitic. They are always talking about him being a Jew in a very negative way, and they accuse him of acting like a "superior Jew." So Robert Cohn has always been the loser in the novel. He may have won a brief victory in a physical fight, but overall, he is a loser.
After the fight with Jake and Mike, Cohn finds Brett and her bullfighter boyfriend, Romero. He then starts a fight with Romero and beats him up pretty badly in front of Brett. Instead of being thankful, Brett calls Cohn an ass, and tries to help Romero. Once again, Cohn wins the physical fight, but the moral battle was over before he even began because he was already a loser in Brett's eyes.