When Emperor Gaozhu of Tang (唐高祖） was reigning, there were a number of battles that were helmed by his second son, Li Shimin（李世民） instead of the crown prince Li Jiancheng(太子李建成), such as:
- The campaign to reunify the empire
- Battle of Hulao
- and so on..
Li Shimin did well in winning the battle, and collected one win/royal title after another, thus greatly expanded his influence and ultimately gave him the confidence and power to slaughter his brother Li Jiancheng ( the crown prince) and Li Yuanji（李元吉）, and forced his father to relinquish the throne in the Xuanwu Gate Incident (玄武门之变） on 2 July 626.
As a seasoned politician, Emperor Gaozhu must have seen this day coming. But my question is, why the Emperor Gaozhu was still insisting on sending Li Shimin into one battle after another despite knowing the consequence of doing so? Could he not let his crown prince Li Jiancheng command instead?
After all, Li Jiancheng was also a great warrior in his own right and participated greatly in Tang's dynasty founding.