신라는 나당연합을 통해 백제·고구려를 멸망시킴으로써 삼국통일을 달성할 수 있었으나, 唐은 나당동맹을 통해 자신들의 팽창전략을 실현하고자 했기에 양국의 충돌은 피할 수 없었다. 신라가 옛 백제지역에 대한 지배를 확립하면서 충돌은 격화되었고, 한강 이북의 옛 고구려 지역을 중심으로 전개되었다. 그러나 강원 영서지역까지 전역이 확대되면서 戰勢가 악화됨에 따라 신라는 영서지역을 위수지역으로 하는 군단의 증설이 필요했다. 나당전쟁의 격화로 전역이 확대되면서 군사적 불확실성이 증폭되자 강원 영서지역 을 위수지역으로 하는 신삼천당 3개 군단이 설치되었다. 우수주(춘천)는 예로부터 중요한 교통로상에 위치했으며, 나당전쟁 동안 신라 동부전선의 전진기지이자 교두보의 위상을 지니고 있었다. 교통로 후방인 내토군(제천)에도 신삼천당이 두어졌는데, 동부전선의 확산 저지와 영서로 방어를 위함이었다. 의림지가 위치한 내토군은 변방을 지키는 병사들을 위한 전방의 중요한 식량공급 기지였으며, 방어와 보급의 이중 기능을 담당했다. 내생군(영월)에도 두어졌는데, 나당전쟁의 공간확대와 더불어 시간확대가 영향을 미친 결과였다. 신라 동부전선의 방어를 위해 창설된 신삼천당은 편제와 운용에 있어 지리적 특성이 고려되었다. 신라 중고기 보병군단으로 존재했던 삼천당을 모범으로 하여 파생된 군단이었다. 병력구성에 있어서는 지휘관의 경우 왕경인과 지방민이 병용되었다. 현지 출신의 군관을 참모나 하급군관 등으로 활용하여 보다 효과적인 작전수행을 할 수 있었으며, 일반병졸의 경우 지방민이 보다 더 활용되었다. 병졸의 충원방식은 자원병이 주축을 이루었는데, 전투력에 있어 적극성을 확보할 수 있었기 때문이었다. 신라 중고기부터 기능했던 삼천당을 기준으로 함과 동시에 진흥왕대 영역확장의 理想을 재현하려던 취지가 반영된 결과이기도 했다.

Silla could attain the unification of three nations by destroying Baekje and Goguryeo through Silla-Tang coalition; however, Tang failed to avoid the collision of two nations as it meant to realize its own expansion strategies through Silla-Tang alliance. The collision got severer as Silla established its ruling over the old Baekje’s area, and this developed around the old Goguryeo’s area on the north of the Han River. But the war was extended even to Yeongseo of Gangwon, and it was transmitted to Silla as well; therefore, it was needed to increase corps that could guard the Yeongseo area. As military uncertainty increased in this way, three Shinsamcheondang corps were installed to guard Yeongseo of Gangwon. Woosuju(Chuncheon) had been located on the crucial traffic route from a long time ago, and it had its status as an advanced base and bridgehead of the eastern front in Silla during the Silla-Tang War. In Naetogun(Jecheon), the back of the traffic route, too, Shinsamcheondang was set, which was to prevent the extension of the eastern front and defend Yeongseo area. Naetogun where Uirimji was located was a crucial food supply base in the front for the soldiers guarding the frontier, and it played dual roles, defense and supply. It was set in Naesaenggun(Yeongwol), too, and it was the result of both spatial and temporal extension of the Silla-Tang War. In founding Shinsamcheondang to defend the eastern front of Silla, geographical characteristics were considered in terms of organization and operation. It was the corps derived by taking as an example Samcheondang which had existed as an infantry corps during the medieval period of Silla. In the organization of the troops, both capital people and local people were appointed as commanders. Local military officers were used as advisors or subordinate military officers, so it was possible to launch operations more effectively. As general soldiers, local people were utilized more. In the modes of recruiting soldiers, volunteer soldiers were the most, which was because it was possible to secure aggressiveness in terms of fighting power by it. This was the result of the intention to take as the reference Samcheondang that had functioned from the medieval period of Silla and also realize the ideal of expanding the area in the reign of King Jinheung.

Shinsamcheondang, Yeongseo area, Woosuju, Naetogun, Naesaenggun