How did pre-modern historians explain Qin's unification of China?

Short Answer

In no particular order, some reasons given by classical Chinese writers include:

• Infighting: the Six States failed to unite against Qin
• Human resources: the Six States threw away their great politicians and generals
• Fate: Qin's rise was divinely ordained, i.e. Mandate of Heaven
• Geography: Qin was advantaged by its defensive terrain
• Reforms: Legalist reforms introduced by Shang Yang.

Note that historically, Chinese historians tended to focus on giving a seemingly factual narrative, punctuated by the occasional comment on a person's character or Confucian morality. A classical preference for conciseness further compounds the lack of in-depth analysis.

In many cases, analysis of the Warring States situation came from people trying to make a point about their contemporary politics via presenting historical examples. For example, the essay by Su Xun was designed to argue against his own Song Dynasty making peace with the nomadic empire of Liao.

Full Answer with Selected Excerpts

Several writers indeed expressed a belief that Qin unified China under the mandate of heaven. For instance, Ming Dynasty scholar Li Zhen argued that all Seven Warring States were about the same, except Qin was fated by heaven to succeed and had the advantage of terrain.

《明·李楨·六國論》向使有擅形便之利如秦者，而又得天助焉。未必不復增一秦也。

If another had the benefits of terrain like Qin, and had the support of Heaven, then we could well have had a second Qin.

He didn't invent this, of course. As early as the Han Dynasty, Sima Qian's pioneering Records of the Grant Historian commented on the demise of Wei, after which Qin conquered the rest of China in quick succession, that:

《漢·司馬遷·史記·魏世家》 說者皆曰魏以不用信陵君故，國削弱至於亡，余以為不然。天方令秦平海內，其業未成，魏雖得阿衡之佐，曷益乎？

Pundits all say that Wei declined and fell because it didn't employ Lord Xinling. I'm not convinced. Heaven mandated Qin would conquer the world. Until that task is completed, even if Wei had the aid of Yi Yin, it wouldn't do any good.

Of course, "heaven commands it" is largely a hand wave and an euphemism for "they got lucky". As that sentence admits, many commentators placed the blame in more concrete places by attributing Wei's downfall to not using Lord Xinling. That's part of a broader trend attributing the demise of the Six States to failing to give the right people power. This is often a result of Qin successes in getting rid of skilled enemy commanders via espionage.

《漢·司馬遷·史記·魏公子列傳》公子率五國之兵破秦軍 ... 秦王患之，乃行金萬斤於魏，求晉鄙客，令毀公子於魏王 ... 秦聞公子死，使蒙驁攻魏，拔二十城，初置東郡。其後秦稍蠶食魏，十八歲而虜魏王，屠大梁。

Lord Xinling led the five armies to crush Qin's forces (west of the Yellow River) ... Fearful, the Qin king bribed a Wei courtier with 1,000 gold pieces to slander Lord Xinling ... Once Lord Xinling was dead, Qin attacked and took sacked 20 cities. 18 years later Wei's capital to the sword.

《漢·司馬遷·史記·廉頗藺相如列傳》 李牧者，趙之北邊良將也。秦多與趙王寵臣郭開金，為反閒，言李牧、司馬尚欲反 ... 趙使人微捕得李牧，斬之。廢司馬尚。後三月，王翦因急擊趙，大破殺趙蔥，虜趙王遷及其將顏聚，遂滅趙。

Li Mu was a great general from the Zhao's northern borders ... Qin gave the Zhao King's favoured courtier gold to say Li Mu and Sima Shang were rebellious ... Zhao captured Li Mu and beheaded him, and dismissed Sima Shang. Three months later, Wang Jian destroyed the Zhao army and captured the King. Thus Zhao fell.

《宋·司馬光·資治通鑑·秦紀二》 齊人怨王建不早與諸侯合從，聽奸人賓客以亡其國

The people of Qi complained that their king did not pursue a coalition with the other states, and that he left the state fell by listening to treacherous courtiers.

Some writers took a broader, more strategic view of the late Warring States period. One factor, which as noted above Li Zhen had mentioned, was Qin's geographical advantages. That is, the Qin heartland consisted of near unassailable defensive terrain, from which Qin forces could raid the Central Plains with impunity. For instance, Tian Ken reportedly told the Han Empire's founding emperor, whose powerbase was in the old Qin lands, that:

《漢·司馬遷·史記·高祖本紀》秦，形勝之國，帶山河之險，縣隔千里，持戟百萬，秦得百二焉。地勢便利，其以下兵於諸候，譬尤高屋之上建瓴水也。

Qin is a country of winning terrain with strategic mountains and rivers. A million pikemen can be held off by 20,000 Qin soldiers. From this great terrain, attacking the lords of the east is (unstoppable) like pouring water from a height.

That Qin had the great fortune of unassailable geography was sort of a conventional wisdom in ancient China. The Han Dynasty political writer, Jia Yi, also opened his treatise on Qin's failure with:

《漢·賈誼·過秦論》 秦孝公據殽函之固，擁雍州之地，君臣固守而窺周室

Duke Xiao of Qin occupied the defensiveness of Mount Yap and the Hangu Pass, and held the province of Yong. He and his ministers turtled and looked to replace the Zhou Dynasty.

Jia Yi went on to credit the legalist reforms of Shang Yang under Duke Xiao for giving Qin the strength to conquer the eastern states.

《漢·賈誼·過秦論》 當是時，商君佐之，內立法度，務耕織，修守戰之備；外連衡而鬥諸侯。於是秦人拱手而取西河之外

Lord Shang assisted him. Domestically, he established laws and encouraged agriculture and weaving, and prepared for war. Externally, he made Horizontal Alliances to pit the states against each other. And thus the people of Qin easily took land east of the Yellow River.

Note that here Jia Yi also touched upon the success of Qin in splitting the Six States up diplomatically. No coherent or lasting coalition was ever formed in the east to challenge Qin hegemony, beyond the occasional battle. During the Song Dynasty, this aspect of Qin's rise became the focus of several writers.

For example, the essayist Su Xun penned what is possibly the most famous classical Chinese treatise on the subject. In it, he argued that the Six States fell because they ceded land to Qin in order to make peace. He believed this weakened the states while strengthening the insatiable Qin. Eventually Qin grew so powerful it could not be resisted by the crippled states.

《宋·蘇洵·六國論》 六國破滅，非兵不利，戰不善，弊在賂秦。賂秦而力虧，破滅之道也 ... 以地事秦，猶抱薪救火，薪不盡，火不滅。

The Six States fell not because their troops were weak or they fought poorly. Their fault lies in bribing Qin with land. Ceding land to Qin weakens their strength, leading them to destruction.

Serving Qin with land is like trying to put out a fire with firewood. The fire does not go out until the wood runs out.

Some writers focused on the infighting by the Six States as the cause of their fall. In this view, Qin's ascendancy can be attributed to the other Six States being too preoccupied killing each other, as opposed to uniting against a common enemy. For instance, the monumental Tzu Chih Tung Chien by Chancellor Sima Guang concluded its coverage of the Qin conquests with:

《宋·司馬光·資治通鑑·秦紀二》 從衡之說雖反覆百端，然大要合從者，六國之利也 ... 向使六國能以信義相親，則秦雖強暴，安得而亡之哉！夫三晉者，齊、楚之籓蔽；齊、楚者，三晉之根柢；形勢相資，表里相依。故以三晉而攻齊、楚，自絕其根柢也；以齊、楚而攻三晉，自撤其籓蔽也。安有撤其籓蔽以媚盜，曰「盜將愛我而不攻」，豈不悖哉！

Although the theory of Vertical and Horizontal Alliances is fickle, generally speaking a Vertical Alliance (uniting the Six States against Qin) is to the Six States' benefit ... if the Six States could treat each other with trust and honour, then even brutal Qin could not conquer them.

The Three Jins (Wei, Zhao, Han) shielded Qi and Chu; Qi and Chu supported the Three Jins. They relied on each other inside and out. Thus the Three Jin's attacks on Qi and Chu were like cutting off their own roots; Qi and Chu's attacks on the Three Jin is like removing their own defences.

How can one remove one's own defences to please a band, and say "now the bandit will leave me alone now because he likes me"? That's ridiculous.

Renowned essayist Su Zhe also echoed this view in his treatise:

《宋·蘇轍·六國論》 夫韓、魏不能獨當秦，而天下之諸侯，藉之以蔽其西，故莫如厚韓親魏以擯秦 ... 以四無事之國，佐當寇之韓、魏，使韓、魏無東顧之憂，而為天下出身以當秦兵 ... 不知出此，而乃貪疆埸尺寸之利，背盟敗約，以自相屠滅，秦兵未出，而天下諸侯已自困矣。至使秦人得伺其隙，以取其國，可不悲哉！

Han and Wei cannot hold Qin back on their own, but the rest of the world could shield behind them. Thus the best course of action was to aid them against Qin ... The four states at peace should have supported Han and Wei on the front lines. Give them no reason to fear their eastern borders, so that they may fight Qin's armies for the whole world.

But instead, they fought over inches of land out of greed, going back on their treaty promises to kill each other. The states were wearied before Qin even attacked, allowing Qin's armies to take advantage of their weakness and annex their countries. This is sad.