What is the fundamental reason for Qin's reunification?

The fundamental reason for Qin's reunification lies in the success of Shang Yang's political reform, which greatly promoted the progress of Qin's productive forces. The secondary reason is that Qin is located in the northwest, which is easy to defend but difficult to attack. Let me talk about the first reason, the success of Shang Yang's political reform. First of all, I don't think Shang Yang's reform can simply be regarded as a feudal reform, because the land system in Qin dynasty has not become private, but it is still state-owned, which is not much different from the land ownership in slave society before. But how did he develop the productive forces? The main reason is that since the mid-Spring and Autumn Period, Qin has expanded its territory to the west, with a vast territory and a sparse population, and the core policy of Shang Yang's political reform is "the war of ploughing fields". Therefore, after the political reform, Qin actively absorbed the Kanto labor force and immigrated to Qin to farm. From the legalist point of view, people do everything out of utilitarian considerations, so they reward farming and develop production through rewards and punishments, so Qin's agricultural production soon surpassed other countries. Obviously, Shang Yang's farming war policy serves the purpose of farming war. Shang Yang believes that if the country does not take the initiative to wage war abroad, on the one hand, it will produce some idle people, which is not conducive to domestic stability and unity, on the other hand, it will consume the limited wealth of the country and corrupt the national atmosphere. Therefore, Qin established a very strong army on the basis of strong agricultural production, and Qin people could only fight hard through farming and production. Turn yourself from the exploited class into the exploited class and master the means of production. We should see that the reform in the Warring States period was mainly led by legalists, and only Qin succeeded, and it was from top to bottom. This is because Qin has been consolidating the centralization of monarchy since the Spring and Autumn Period, and there are not many patriarchal relations left in Qin, so clan forces cannot manipulate the monarch. The policy of Shang Yang's political reform protected the monarch's rights to the maximum extent, so the political reform could be carried out. Look at the geographical location of Qin State. The Guanzhong Plain where Qin State is located, to its east, is a long and narrow culvert, and to its southeast, is the military attache road in the crevice of Qinling Mountains. Qin borders Wei and Chu respectively on these two sides. At the beginning of Shang Yang's political reform, Qin first controlled the original "nail house" of Wei in Hexi area by fraudulent means, thus completely grasping the only channel for Qin Xiangdong-the other. Through diplomatic fraud and military strikes, Qin completely drove the Chu army out of southern Shaanxi, thus completely controlling Wuguan Road, and Qin completely occupied the geographical advantage.