During the Warring States Period, there was a situation where the Seven Heroes competed for hegemony, but in the end, the other six countries were destroyed by the Qin State that rose after the 'Shang Dynasty Reform', what is the reason for the decline of these six countries?
Let's take a brief look at each of them
1, Qi State. 
The State of Qi began to compete for hegemony with the Duke of Qi Huan in the Spring and Autumn Period, and it was a great power at that time, and in the Warring States Period, the Tian clan replaced Qi, and the State of Qi began to become strong again.
Especially during the reign of King Qi Wei, King Xuan of Qi, and King Qi Yu, almost all of Qi were the overlords of the East.
However, when the 'Five Kingdoms Expedition' occurred during the period of King Qi, the State of Qi was almost on the verge of extinction, and later King Xiang of Qi restored the country, which gradually made the national strength of the State of Qi begin to recover slowly.
However, after the death of King Qi Xiang, the ruling Queen Qi Xiang began to implement the policy of closing the country, so that although the Qi State retained its strength for a while, it became a frog at the bottom of the well, and was finally destroyed by the Qin State.
Therefore, the main reason for the decline of the Qi State is actually the defeat of King Qi and the retreat of Queen Qi Xiang.
2, Chu State. 
Since the Spring and Autumn Period, the state of Chu has become the hegemon of the south, and its national strength has hardly been weakened, but in the early Warring States period, a series of civil strife occurred in the state of Chu, which made the national strength stagnate.
It was not until King Chu Su began to recuperate after succeeding to the throne, and experienced King Chu Xuan, and during the period of King Chu Wei, the national strength of Chu State became strong again, and it almost became the second overlord of the Seven Heroes after Wei at that time.
However, in the period of King Chu Huai, due to a series of decision-making mistakes, after the Battle of Danyang and the Battle of Lisha, King Chu Huai was captured, resulting in serious losses in the Chu State and a great loss of national strength.
However, after King Xiang of Chu succeeded to the throne, there were also serious mistakes in political and diplomatic decision-making, which caused the capital of Chu to fall and was forced to move the capital.
After moving the capital, although the state of Chu also had a flash of inspiration during the Chunshen monarchy, it could not withstand the decline of national strength and the frequent civil strife after its death.
Therefore, the reason for the decline of the Chu State is mainly due to the serious mistakes in the decision-making of the two generations of Chu kings, King Huai of Chu and King Qingxiang of Chu, and the serious civil strife in the later period of the Chu State, which led to the complete decline of the Chu State.
3, Wei State. 
Wei was the first powerful and hegemonic country in the Warring States period after the division of the three families.
After experiencing the strength of the three generations of Wei Wenhou, Wei Wuhou, and Wei Hui Wang, the Wei State began to go downhill in the late Wei Hui Dynasty.
The first step in the decline of the Wei State was that King Wei Hui was too arrogant at that time, thinking that he could destroy the powerful Korea at that time by relying only on the strength of his own Wei State, so he had a three-year-long life-and-death battle with South Korea 'Xinzheng Battle'.
The result can be imagined, naturally Wei and Han fought to defeat both, giving the surrounding Qi State, Qin State, and Chu State the opportunity to benefit.
After this war, the Wei State rapidly declined under the situation of the surrounding countries, and it was not until later that King Wei Hui adopted the strategy of 'Lianheng' and formed alliances with the surrounding countries, and the Wei State began to slowly recover during the reign of King Wei Xiang.
However, by the time of King Zhao of Wei, due to the strong rise of Qin, Wei was forced to fight a major war with Qin in Yique with Korea.
Due to the defeat in the Battle of Yique, a large amount of land was lost, and Wei was completely reduced to a second-rate state.
Although the Wei State later reaped the greatest benefits in the Five Kingdoms War, it was not worth being ravaged by the Qin army's iron cavalry.
Although by the time of Wei Gongzi Wuji leading the troops, there was a flash of inspiration in the Wei State, but it was no longer possible to reverse the situation that the Wei State had already begun to decline and perish.
Therefore, the reason for the decline of Wei was mainly due to the serious loss of the Xinzheng battle with Korea, and then the decline caused by the complete inability to recover from the battle of Yique.
4. South Korea. 
Korea and Wei are both from the state of the Three Jin Dynasty, and they can be regarded as a pair of brothers who love and kill each other.
After all, the early rise of South Korea was indispensable to the leadership of the big brother of Wei.
However, after Korea gradually became stronger, it also began to break with Wei after Zhao.
Especially after Shen Buxiao's change of law, Korea's national strength increased greatly, and almost single-handedly repelled the allied forces of Qi, Chu, and Song for Wei.
It was because of the victory in this 'Battle of Xiangling' that Han Zhaohou was extremely arrogant and had begun to ignore the overlord Wei at that time.
The biggest war after the break between Korea and Wei was the protracted and three-year-long 'Battle of Xinzheng'.
It was because of this battle that Han Zhaohou completely abolished his national strength, and in the next few years, like Wei, he suffered from the military disasters of Qin and Chu, which caused great damage to the vitality of Korea.
Later, Han Zhaohou learned a lesson and began to recuperate, and it was only during the reign of King Han Xuanhui that Korea's national strength began to slowly recover.
Because South Korea failed to participate in the invasion of Qin by the Five Kingdoms, it suffered serious losses, and then South Korea began to play the foreign policy of 'Chaoqin Muqi', but this did not bring too much benefit to South Korea, but the land was constantly encroached upon by Qin, and even the old capital Yiyang was lost.
In the end, Korea was defeated in a 'Battle of Yique' that united Wei and Qin, and went into decline completely, and Korea became almost the 'little brother' of Qin, using the diplomacy of 'Shuzhi' to waver between the Six Kingdoms and Qin until it was destroyed by Qin.
Therefore, the decline of Korea was mainly caused by the defeat of its two major wars, the 'Battle of Sinjeong' and the 'Battle of Ique', and the diplomacy of 'Shuji'.
5, Zhao Guo. 
Although Zhao was from the same state of the Three Jin Dynasty as Wei and Han, since the break with Wei, Zhao was more independent than Wei and Han.
The main reason is also because of the environment in which Zhao is located, after all, the north of Zhao has to face Rong Di, and to participate in the Central Plains hegemony, it has to face the two powerful countries of Wei and Qi.
Zhao was still very strong in the early stage of the Three Jin Dynasty, almost second only to Wei, but because of many years of war with Wei, until Handan was broken by the Wei army, Zhao was also greatly injured.
After the succession of Marquis Suhou of Zhao, the national strength of Zhao was gradually restored, until the period of King Wuling of Zhao, Zhao began to rise strongly, annexing a large amount of land in Zhongshan, Yunzhong and Jiuyuan.
With the 'Dune Palace Change' and the failure to compete with Qi for the Song land, the national strength of Zhao was greatly damaged.
Until Qi and Chu declined one after another, the state of Zhao, which gradually recovered its vitality, became strong again and became the most powerful country of the six eastern countries at that time.
However, because Zhao was afraid of the powerful Qin State at that time, he did not carry the 'banner of unity', but made enemies on all sides diplomatically, and finally lost completely in a life-and-death battle with Qin State, the Battle of Changping, and began to collapse.
Therefore, the decline of the Zhao State actually began with the 'Battle of Zhao Qi' that took place for the Song Land and the Qi State, until the later 'Battle of Changping' completely declined.
6, Yan Guo. 
Among the Seven Heroes of the Warring States, Yan is almost the country with the lowest sense of existence.
In the Warring States Period, it is almost in the period of 'King of the Five Kingdoms' and 'The First Five Kingdoms Expedition to Qin'.
But the event that brought Yan into the public's eyes the most was the incident in which King Yan gave the throne to the prime minister of the country.
It was because of this incident that civil strife broke out in the Yan State, and the surrounding vassal states began to interfere in the internal affairs of the Yan State, and the Qi State took the opportunity to occupy the Yan State.
Subsequently, the Three Jin Dynasty supported King Yan Zhao to succeed to the throne, and the Yan Kingdom began to revivenation.
After years of recuperation and preparation for war, Yan became a blockbuster in the 'Five Kingdoms Invasion of Qi' war, took revenge on Qi and occupied almost all the land of Qi, making Yan a conspicuous bag among the princes at that time.
But after all, Yan Guo was deep in the bitter cold land at that time, and its development was not as good as that of other countries in the Central Plains, even after occupying the rich Qi State, it did not bring too many benefits to Yan State, but instead became a burden on Yan State at that time.
Therefore, after King Yan Hui succeeded to the throne, he also understood that he could not achieve a long-term occupation of Qi with his own national strength of Yan State, and if this continued, Yan State could no longer bear it, so King Yan Hui began to plan to withdraw his troops, and replaced the main general Le Yi who insisted on occupying Qi State with cavalry.
This naturally led to an army mutiny, and Le Yi defeated the cavalry army with his Yan army with loyalty, and Qi Guotian took the opportunity to start recovering the country.
Originally, Le Yi's peaceful withdrawal of the army could also enable the Yan State to preserve its strength and continue to develop, but because of this turmoil, not only did the Yan army have internal strife, but also were defeated by the Qi army and fled, and the strength of the Yan State was greatly damaged.
After that, there were many years of civil strife in the Yan State, coupled with the constant wars with the Zhao State, which directly caused the Yan State to never cheer up again.
If the Yan State did not be greedy to occupy the Qi State after the invasion of Qi, and chose to set up a puppet Qi monarch, it would not have ended up like this.
Le Yi is definitely a first-class famous general in the Warring States Period, but the author is only analyzing from the position of Yan Guo, I hope you understand.
Therefore, the decline of the Yan Kingdom is mainly due to the failure to properly handle the affairs of Qi after this 'Battle of Qi', which led to his 'loss of his wife and his army', and has since gone into decline.