When Sun Jun took the throne, the chaos in China was even worse than it had been during Sun Quan's reign. With the collapse of Cao Wei and Shu Han, the land was controlled by various warlords who had emerged from those kingdoms. Sun Jun's main concern were the warlords Zhang Da and Fang Jiang, who were allied with Sun Quan but attempted to seize land from Eastern Wu after his death. Sun Jun attempted to satisfy them with diplomatic trade agreements, with mixed success.
At the time, he was considered a very weak ruler whose inaction caused the collapse of Eastern Wu. However, it is likely that his unwillingness to attack his rivals was due to his focus on domestic issues within the kingdom. Most historians believe that there was little Sun Jun could have done to save the kingdom.