Note: It was the Treaty of Shimonoseki in which China surrendered its sovereign rights over Taiwan forever.
The argument goes that because the Treaty of Shimonoseki was a treaty imposed on a weaker state (China) by a stronger state (Japan), then the treaty is not valid nor is it legally binding. Hogwash!
One would be hard-pressed to find treaties before the twentieth century what did NOT fit this description. The examples are to numerous to name. Many legal commentators have discussed this phenomenon in history.
As early as 1625, Dutchman Hug Grottos commented that a stronger state imposing its will on a weaker one via a treaty is a legally binding arrangement on both of the powers. German Samuel Pufendorf (1672) and Swiss Emmerich de Mattel (1758) shared these views. Ninteenth century scholars maintained this traditional line of thought. Oppenheim (considered the Webster's of International Law) stated that any repudiation of a treaty violates international treaty law. Little or no distinction was made between defensive and aggressive wars. International practice of the time bears out what these scholars posited.
Even as late as 1931, Dane Azel Moller argued that "war or threat of war, does not, according to positive international law, generally render a treaty invalid." This was in 1931, 36 years AFTER Shimonoseki was signed. Clearly, it was considered leval even at this time. Fellow Dane Julius Hatscheck (1930), as well as Americans Charles Butler, Edgar Turlington, and Charles Hyde argued that the traditional rule still prevailed.
Brazil proposed an amendment to the League of Nations Covenant in 1921. The proposed amendment would have voided any international treaty that violated clauses prohibiting aggression. It is clear that such treaties were still considered legal, else why have such an amendment. The amendment was never adopted.
Even in 1937, the Permanent Court of International Justice, in the River Meuse Case, indicated that the provisions of imposed treaties were indeed valid. This idea in international law would not change until after World War II.
It is clear that international law accepted such imposed, unequal treaties until at least 1937, 42 years after the Treaty of Shimonoseki. Even further, the practices of both the Republic of China AND the People's Republic of China clearly show that even they acknowledge that such treaties were legally binding on the Chinese state. The Republic of China did not unilaterally abrogate any imposed treaty with a Western Power. All were modified through bilateral negotiations. Even the People's Republic of China realized that such treaties (i.e. that gave Hong Kong to Britain, and large tracts of northeastern Asia to Russia) were legally valid and did not attempt to unilaterally abrogate these treaties. It was a treaty that resulted in the return of Hong Kong to Chinese control in 1997.
Another argument that is made is that when the ROC declared war on Japan in 1941, the provision of the declaration that unlaterally abrogated the treaty meant that the treaty was no longer in force. This is a rediculous argument. According to positive international law, no country can unilaterally abrogate a legally binding treaty unless such a treaty has a provision for this abrogation. The Treaty of Shimonoseki had no such provision.
It is obvious that the Treaty of Shimonoseki was a lawful treaty. One may make arguments as to its morality, but its legality can not be disputed. China DID give up Taiwan in perpetuity (forever) in 1895, and there is nothing that can dispute this.