You must not know how laws work. Kashmir agreed to join India in 1948. It was given a special status, enshrined in Article 370 of the Constitution of India. Now, that "special status" has always bothered a lot of Indians; what's so special about Kashmir? Why not any of the other 537 states that formed India in 1947?
There were certain conditions on which Article 370 could be removed. Those conditions were met in 2019 and the Central Government took the decision to remove Article 370 to much rejoicing in areas like Ladakh and Jammu. This was challenged in the Indian Supreme Court (the final arbiter of things Constitutional) and found to be valid.