A blogger I follow via FB, Carrick Ryan (ex Australian security service agent) puts it thus,
While it was the British that implemented partition, it was the Muslim League that demanded it. It was against the wishes of Gandhi, a man executed by a Hindu extremist, and against the counsel of the British, albeit after centuries of "divide and conquer" colonial control.
It was one of the most violent events in history. Neighbours murdered each other the moment they felt legally absolved to do so, communities that had lived side by side for centuries butchered each other for no reason other than religious animosity.
As many as 2 million were killed, 100,000 women were raped and abducted, and as many as 18 million were displaced from religiously motivated massacres by Muslims, Hindus, and Sikhs alike.
Pakistanis and Bangladeshis don't share an ethnicity or language, or a border, but they share their religion, and that was enough to initially be seen as the same nationality. Their religion was their nationality.
Ten weeks after independence, India and Pakistan went to war for the first time. It's happened three more times since then.
When the people of Kashmir were offered membership of either Pakistan or India, the Muslim majority wanted to join Pakistan. The Hindu Raj chose to join India. The Muslims of Kashmir have demanded this decision be reversed ever since.
A passionate allegiance to a political entity they've never been a part of, based on no other value than religion, strong enough to wage war.
Make no mistake, this is a holy war. A country that has Islam enshrined into its constitution and a country with a Hindu nationalist Prime Minister are threatening to annihilate one another, and the crowds are getting loud.