Kashmir was (and still is) a Muslim majority state of undivided India in 1947 ruled by a Hindu prince Maharaja Hari Singh. The Kashmir crisis started along with the signing of accession accord with the Republic of India by Hari Singh.
There were Pakistani invasions and Chinese claim of Kashmir in 1947 which split Kashmir into three parts.
- Pakistani administered Kashmir is known as Azad Kashmir (plus, Gilgit-Baltistan and part of Siachen Glacier)
- Chinese part of Kashmir is known as Aksai Chin (plus, Shaksgam Valley)
- Indian administered Kashmir is known as Jammu and Kashmir (plus, part of Siachen Glacier).
Note that, Gilgit-Baltistan joined Pakistan voluntarily. Pakistan Army took control of this area later. Azad Kashmir was occupied by Pakistan by invasion.
Grabbing Kashmir is a part of a greater plan of rebuilding Akhand Bharat.
Akhand Bharat is the ideology of the Indian Hindu extremist group RSS with which BJP holds the common ground.
The main binding force of India is the Hindu nationalism or Hindutva which originated during the British rule at the start of 19th century. Note that, because of the Napoleonic wars, the Europe saw the rise of Nationalism. Indian people, as part of the citizens of the British empire, started to go to Europe for higher education. The Western education gave rise to Indian Nationalism.
Hinduism played a significant role in this regard. As Muslims were outsiders and invaders, it was convenient to find a hero, who is indigenous and can be taken as a symbol of Indian unity. That hero was Emperor Bharata who ruled Akhand Bharat during the Vedic era. There was also a historical analog of Bharata. Ashoka Maurya unified the entire India and expanded India to its greatest extent as well.
As the Wikipedia suggests,
India has been unified under many emperors and governments in history.
Ancient texts mention India under emperor Bharata and Akhand Bharat,
these regions roughly form the entities of modern-day greater India.
The Mauryan Empire was the first to unite all of India and South Asia
(including much of Afghanistan).
Grabbing Kashmir will humiliate and debilitate India's last enemy in South Asia and the only arch-enemy in the world Pakistan, and hopefully, begin the start of a new foundation of Akhand Bharat.
Now, the relevant question is: What about the other parts of Akhand Bharat?
Well, India almost at the control of the Akhand Bharat except for its arch-enemy Pakistan. Grabbing landmasses is not the only way to build empires. Creating hegemony is another tool.
We can take the examples of Russia's Putin and Turkey's Erdogan. Putin and Erdogan are reportedly obsessed with the revival of their respective empires (e.g. Russian/Soviet Empire and Ottoman Empire).