hmmm... i always thought Sun Jian that time already occupy 江东, i guess not, lol :p:p:p
Sun Jian family was originally from Jiangdong. He joined the alliance against Cao Cao as equal ally to Yuan Shu etc. under Yuan Shao as alliance leader. His condition, if the alliance beat Cao Cao, was to have the Jiangdong land. That's why his son Sun Ce was so bent on Jiangdong.
Too bad Sun Jian got defeated and killed in battle. Actually, he was sabotaged by Yuan Shu who halted his supply lines as an ally should do. These internal power struggles and sabotages amongst allied troops contributed much to Cao Cao's ultimate victory at Guandu.
It's also lengendary that Sun Jian met his end through his own oath. When Yuan Shao as alliance leader asked him to hand over the Imperial Seal, Sun Jian swore that he didn't have it, otherwise would die in a rain of arrows.
Sun Ce ultimately handed over the Imperial Seal in exchange for troops, between 3,000 to 5,000 amongst varying records. In any case, plus his own loyalist officers and troops, he couldn't have more than 10,000 to begin with. Zhou Yu (his sworn brother since schooling days) was wandering around aimlessly with a small private army amidst the chaos then.
Zhou Yu's reputation in Jiangdong was very good as a gentleman, scholar and warrior. He helped Sun Ce recruited even more troops along the way of entry into Jiangdong. Sun Ce practically took Jiangdong without any serious resistance from the minor warlords there. By the time he's done, the state of Wu was formed and he had more than 50,000 troops to defend it.
That's why Zhou Yu's paramount position in command of the Wu army could never be questioned, even by Sun Quan after Sun Ce's death.