Hemingway foreshadows the brawl between Mike, Jake, and Robert at the very beginning of the book, when he introduces Robert as an accomplished fighter in Chapter 1:
Robert Cohn was once middleweight boxing champion of Princeton. [...] He cared nothing for boxing, in fact he disliked it, but he learned it painfully and thoroughly to counteract the feeling of inferiority and shyness he had felt on being treated as a Jew at Princeton.
There is frequent tension between Robert, Brett, Mike, and Jake throughout The Sun Also Rises. While Robert's incessant pursuit of Brett reflects his deep and old insecurities, his continued verbal abuse at the hands of Mike reflects Robert’s perceived "otherness" to the group as a Jewish person. When the tension between Mike and Robert eventually comes to a head and an altercation results, Hemingway has already prepared the reader for what happens, since the very beginning of the book underscores Robert's talent for boxing and his tendency to use this skill as a way to combat ignorance and intolerance. Using his experience from his days in the ring, then, he sets both Mike and Jake down on the pavement—an outcome that has almost come to seem inevitable.
As Brett begins to fall for Pedro Romero, and as Mike's antagonization of Cohn becomes more and more vicious, the temperament of the group in Pamplona grows increasingly strained and increasingly volatile. The beginning of Chapter 16 brings a major shift in the weather for the first time in the novel, thus foreshadowing the more turbulent events that are still to come:
In the morning it was raining. A fog had come over the mountains form the sea. You could not see the tops of the mountains. The plateau was dull and gloomy, and the shapes of the trees and the houses were changed. I walked out beyond the town to look at the weather. The bad weather was coming over the mountains from the sea.
Sure enough, as the ominous weather foreshadows a storm, there comes an especially brutal outburst from Mike towards Cohn and an outburst of intense anguish from Brett over her growing love for Romero. In particular, the word "gloomy" hints at this change, as does the general image of heavy fog obscuring mountaintops, which creates an ominous quality in the narrative.