Yang Tong sibling Emperor Gong of Sui
During Emperor Yang's reign
It was not recorded when Yang Dong was born, except that his elder brother Yang Tan (楊倓) was born in 603 and his younger brother Yang You was born in 605.  He was the second of three sons of Yang Zhao, Emperor Yang's son and crown prince.  His mother was Yang Zhao's concubine, Consort Liu.
In 606, Yang Zhao died.  According to Confucian principles of succession, Yang Dong's younger brother Yang You, would have been considered Yang Zhao's heir and successor, as Yang You was born of Yang Zhao's wife Crown Princess Wei.  However, Emperor Yang did not create any son of Yang Zhao crown prince to replace Yang Zhao, but left the matters of succession ambiguous between them and Yang Zhao's younger brother Yang Jian the Prince of Qi.  He did, however, create Yang Dong and his brothers imperial princes, and Yang Dong was created the Prince of Yue.
In spring 613, when Emperor Yang launched his second of three campaigns against Goguryeo, he left the eight-year-old Yang Dong nominally in charge of the eastern capital Luoyang, with the official Fan Zigai (樊子蓋) actually responsible.  Subsequently, while Emperor Yang was in Goguryeo territory, the general Yang Xuangan rebelled near Luoyang, and Fan defended Luoyang under Yang Dong's command.  The general Wei Wensheng (衛文昇), leading the army from the capital Chang'an under Yang You's command, came to Luoyang's aid, and Emperor Yang also abandoned the Goguryeo campaign and sent the generals Yuwen Shu and Lai Hu'er (來護兒) back to the Luoyang region; these Sui generals together defeated Yang Xuangan.
In 616, with most of Sui territory, particularly the northern commanderies, engulfed in agrarian rebellions, Emperor Yang went from Luoyang to Jiangdu (江都, in modern Yangzhou, Jiangsu), leaving Yang Dong in charge of Luoyang assisted by the officials Duan Da (段達), Yuan Wendu (元文都), Wei Jin (韋津), Huangfu Wuyi (皇甫無逸), and Lu Chu (盧楚). The rebel leaders Li Mi and Zhai Rang soon took advantage of Emperor Yang's departure (along with the elite Xiaoguo Army (驍果)), to capture the food storages Luokou Storage (洛口倉) and Huiluo Storage (回洛倉), near Luoyang, causing Luoyang to be down on food supplies.  In spring 617, Yang Dong sent the generals Liu Changgong (劉長恭) and Pei Renji (裴仁基) against Li Mi and Zhai, but Liu and Pei were defeated.  In summer 617, with his forces repeatedly defeated by Li Mi's, Yang Dong sent the official Yuan Shanda (元善達) to Jiangdu to seek aid from Emperor Yang, but Emperor Yang, believing in the prime minister Yu Shiji's assessments that the situation was not as severe as Yuan Shanda was claiming, initially refused to send aid.  Pei soon surrendered to Li Mi, making Luoyang's position even more precarious.  Emperor Yang finally did order the generals Pang Yu (龐玉) and Huo Shiju (霍世舉) to lead the troops from the Chang'an region to aid Luoyang, and Pang and Huo were able to force Li Mi away from Huiluo, allowing Luoyang to regain some of its food supply, although by fall 617 Li Mi had recaptured Huiluo.
In fall 617, Emperor Yang sent the general Wang Shichong (from Jiangdu) and several generals in other outlying areas to lead their troops to aid Luoyang.  Wang was able to stem Li Mi's advances, and the armies stalemated.  Meanwhile, the general Li Yuan had rebelled at Taiyuan and soon captured Chang'an, declaring Yang Dong's brother Yang You emperor (as Emperor Gong).  In spring 618, Li Yuan sent his sons Li Jiancheng and Li Shimin to lead an army to Luoyang, ostensibly to aid it, but Yang Dong and his officials chose to have no communications at all with Li Jiancheng and Li Shimin.  After Li Jiancheng and Li Shimin briefly engaged Li Mi, they considered the idea of attacking Luoyang but did not do so, and instead withdrew to Chang'an.
In late spring 618, Emperor Yang was killed in a coup led by the general Yuwen Huaji, who declared Emperor Yang's nephew Yang Hao the Prince of Qin emperor, and began to lead the Xiaoguo Army on a trek back north.  Soon, news of Emperor Yang's death arrived at Chang'an and Luoyang.  Li Yuan, in response, had Yang You yield the throne to him, establishing Tang dynasty as its Emperor Gaozu.  The officials at Luoyang declared Yang Dong emperor, and those commanderies still loyal to Sui recognized him as emperor as well.
brother or sister kin