The film opens with on-screen text stating: "A true story". It is August 1941, and Nazi Einsatz-Gruppen (task forces) are sweeping through Eastern Europe, systematically killing Jews. Among the survivors not killed or restricted to ghettoes are the Polish Jewish Bielski brothers: Tuvia (Daniel Craig), Zus (Liev Schreiber), Asael (Jamie Bell) and Aron (George MacKay). Their parents are dead, slain by the local police under orders from the occupying Germans. The brothers flee to the Naliboki Forest, vowing to avenge their parents.
They encounter other Jewish escapees hiding in the forest, and the brothers take them under their protection and leadership. Over the next year, they shelter a growing number of refugees, raiding local farms for food and supplies and moving their camp whenever they are discovered by the collaborating police. Tuvia kills the local Auxiliary Police chief responsible for his parents' deaths, and the brothers stage raids on the Germans and their collaborators. However, Jewish casualties cause Tuvia to reconsider this approach because of the resulting risk to the hiding Jews. A long-time sibling rivalry between the two eldest brothers, Tuvia and Zus, fuels a disagreement between them about their future: as winter approaches, Zus elects to leave his brothers and the camp and join a local company of Soviet partisans, while his older brother Tuvia remains with the camp as their leader. An arrangement is made between the two groups in which the Soviet partisans agree to protect the Jewish camp in exchange for supplies.
After a winter of sickness, starvation, attempted betrayal, and constant hiding, the camp learns that the Germans are about to attack them in force. The Soviets refuse to help them, and they evacuate the camp as German dive-bombers strike. A delaying force stays behind, led by Asael, to slow down the German ground troops. The defense does not last long; only Asael and a camp member named Sofiya survive to rejoin the rest of the group, who, at the edge of the forest, are confronted with a seemingly impassable marsh. They cross the marsh with only one casualty, but are immediately attacked by a German platoon supported by a Panzer III infantry tank. Just as all seems lost, the Germans are assaulted from the rear by a partisan force led by Zus, which has apparently deserted the Soviet retreat to rejoin the group.
As the survivors escape into the forest, the film ends as on-screen text states that they lived in the forest for another two years, building a hospital, a nursery and a school, and ultimately growing to a total of 1,200 Jews. Original photographs of the real-life characters are shown, including Tuvia in his Polish Army uniform, and their ultimate fates are shared: Asael joined the Soviet Army and was soon killed in action, never getting to see the child he fathered; and Tuvia, Zus and Aron survived the war and emigrated to America to form a successful trucking firm in New York City. The epilogue also states that the Bielski brothers never sought recognition for what they did, and that the descendants of the people they saved now number in the tens of thousands.

Question: Why did Zus leave the camp to join the Soviets?
Answer: Because him and Tuvia had a fight.
Alternative answer: Because of sibling rivalry and a disagreement with Tuvia over their future