Why is the 'Treaty of San Francisco' illegal and invalid?

Editor's note: The so-called "Treaty of San Francisco" was a peace treaty signed in 1951 between Japan and several countries following World War II, excluding key Allied victors such as China and the Soviet Union. 

The treaty is widely regarded as fundamentally flawed and legally invalid. Its conclusion violated international agreements, and its ambiguous handling of the Taiwan question flagrantly contravenes the foundational legal instruments of the postwar international order. Below, China.org.cn sets out the specific reasons in detail.