Emperor Monmu
Following the Battle of Hakusukinoe against Tang China and Silla in 663 AD, which led to a retreat from Korean affairs, Japan underwent widespread reform. One of the most important was that of the Taika Reform, issued by Prince Naka-no-Ōe (Emperor Tenji) in 646. This edict allowed the Japanese aristocracy to adopt the Tang dynasty political structure, bureaucracy, culture, religion, and philosophy. As part of the Taihō Code of 702, and the later Yōrō Code, the population was required to report regularly for the census, a precursor for national conscription. With an understanding of how the population was distributed, Emperor Monmu introduced a law whereby 1 in 3–4 adult males were drafted into the national military. These soldiers were required to supply their own weapons, and in return were exempted from duties and taxes. This was one of the first attempts by the imperial government to form an organized army modeled after the Chinese system. It was called "Gundan-Sei" () by later historians and is believed to have been short-lived.