Most of the existing research has argued that the primary cause of the defeat of Old Joseon in the
warfare between Old Jeseon and Yan is because of the weapon system of Yan, such as the iron weapons
and the cavalry. However, comparatively various iron weapons of Yan appeared in the end of the fourth
century BC or third century BC. Moreover, because the production efficiency of iron weapons of Yan was
very low, large scale production of iron weapons was impossible. The proportion of cavalry of Yan was
insignificant too. Therefore, it is difficult to say the iron weapons and the cavalry tactics of Yan gave a
significant impact on the outcome of the warfare. Since the Spring and Autumn period Old Joseon had
established a close relationship with Qi that was one of the hegemonic powers of in the mid Warring
States Period. Qi and Yan were hostile to each other. But Old Jeseon had downplayed the military power
of Yan, since 316 BC Yan had been weakened because of civil war and foreign occupations. And Old
Joseon failed to close military cooperation with Qi. As a result, the primary cause of the defeat of Old
Joseon is thought that the failure of foreign policy.