It is true that chimpanzees, particularly when traveling in small, compatible groups, may maintain peaceful relationships for hours or days. Nevertheless, they can easily be roused to sudden violence, particularly during social excitement. While most fights do not lead to wounding, some certainly do-particularly those directed at individuals of neighboring social groups.
In December 1963, Goliath arrives in camp, late one evening. Every so often he stands upright to stare back in the direction from which he has come. He seems nervous and startles at every sound. Six minutes later three adult males appear on one of the trails leading to camp; one is high-ranking Hugh. They pause, hair on end, then abruptly charge toward Goliath. But he has vanished silently into the bushes on the far side of the clearing. For the next five minutes the three crash about the undergrowth, searching for the runaway, then they emerge and are given bananas. As they sit feeding, Goliath's head peeps out from behind a big tree trunk, some distance up the opposite slope. He quickly ducks back into hiding when one of the three looks up. They all sleep nearby; we hear them call a few times during the night.
Early next morning Hugh returns to camp with his two companions. A few minutes later, Goliath charges down, dragging a huge branch. To our amazement he runs straight at Hugh and attacks him. The two big males fight, rolling over, grappling and hitting each other. It is not until the battle is already in progress that we realize why Goliath, so fearful the evening before, is suddenly so brave today: we hear the deep pant-hoots of David Greybeard. He appears from the undergrowth and displays in his slow, magnificent way around the combatants. He must have joined Goliath late the evening before, and even though he does not actually join in the fight, his presence provides moral support. Suddenly Goliath leaps right onto Hugh, grabbing the hair of his shoulders, pounding on his back with both feet. Hugh gives up; he manages to pull away and runs off, screaming and defeated.
Source: Patterns of Behavior, Chapter 12 Introduction.