The 9th Amendment Act of 1960 adjusted the Indian territory based on agreements with Pakistan.
After the Nehru-Noon agreement was signed between India and Pakistan to divide the territory of Berubari Union, the Government of West Bengal opposed it.
After this Union referred the matter to SC which held that the power of Parliament to diminish the area of a state (under Article 3) does not cover cession of Indian territory to a foreign country.
Hence, Indian territory can be ceded to a foreign state only by amending the Constitution under Article 368. Consequently, the 9th Constitutional Amendment Act (1960) was enacted.