a ruler during the Tang dynasty, conquered many lands, reformed the military, and created law codes. (Taizong/Xuanzong)

The ruler that during the Tang dynasty, conquered many lands, reformed the military, and created law codes. was:

Taizong.

Explanation:

Taizong or Emperor Taizong of Tang with the personal name of Li Shimin was a Chinese emperor that defeated the Tang dynasty enemies and established its power over china. His reign is called the golden era of the Tang dynasty, as well as the exemplary emperor, ideal empire, and the rule with which the rulers that came after him were measured to value them and their reigns.