Why did China claim much of the area that became Arunachal Pradesh after India gained independence?

After India gained independence in 1947, China claimed much of the area that became Arunachal Pradesh primarily because it never recognized the McMahon Line as a legitimate border.12

The dispute stems from the 1913–14 Shimla Convention, where representatives from the British Empire, Tibet, and China met to negotiate boundaries. While the British and Tibetans agreed to the McMahon Line, the Chinese government refused to sign the convention, arguing that Tibet lacked the authority to enter into treaties and that the line was a product of British "imperialist aggression".12

Following the establishment of the People’s Republic of China in 1949, the Chinese government maintained that the line was illegal. China further supported its claims by citing historical maps—including a 1929 edition of Encyclopædia Britannica—that depicted the region as part of China. Conversely, India maintains that the McMahon Line serves as the valid geographic and administrative boundary, a position supported by historical British and Indian mapping.21