Events after the end of British rule in India in August 1947 were momentous: two new countries were created to form predominantly Muslim West and East Pakistan (now Bangladesh) with Hindu-majority India wedged in between.
It was one of the world's largest mass migrations
The break-up along religious lines resulted in the movement of about 14.5 million people - Muslims going to Pakistan from India and Hindus and Sikhs going in the opposite direction.
The new governments were ill-equipped to deal with such a massive migration - one of the largest of its kind in the world - and there was huge violence on both sides of the border.
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The upheaval resulted in a breakdown of law and order: estimates of casualties vary, from between 200,000 up to a million people. Around 12 million people were left homeless and thousands were raped.
Who was to blame?
The British were accused of pulling out of India too quickly. Critics say that they failed to come up with a definitive map of the border, and failed to plan for the huge migration.
Britain argued it was forced to act speedily because of the breakdown of law and order, and that matters would have got worse the longer they remained.
Britain also argued that it had limited resources after World War II.
The two countries - already bitterly divided by the Kashmir question (see below) pursued differing alliances around the world.
India looked to Soviet Russia as its strategic ally, and did not liberalise its economy until the early 1990s.
Pakistan chose China and the US as its key foreign policy partners. But unlike India - which has had political stability apart from the state of emergency declared by Prime Minister Indira Gandhi in 1975 - Pakistan for most of its existence has been governed by the military.
The Kashmir question
The dispute over the Himalayan territory of Kashmir has been the spark for two of India and Pakistan's three wars.
Kashmir had a Muslim majority but a Hindu princely ruler who eventually acceded to India in return for military aid.
Within months of independence, India and Pakistan were at war in Kashmir and the sense of conflict has lingered ever since.
The nuclear issue
Nuclear tests by India in May 1998 and by Pakistan just weeks later provoked international concern.
India and Pakistan have had a rocky relationship over the years
In May 1999, there were fears of nuclear war between the two countries after Pakistani-backed forces entered a mountainous area of Indian-administered Kashmir.
In 2004, the leading Pakistani nuclear scientist, AQ Khan, confessed to selling nuclear weapons technology to Libya, Iran and North Korea.
India is tipped to be a 21st Century superpower with a population that will overtake China's by 2050.
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India's economy has seen dramatic growth and much foreign investment.
Pakistan too has had recent economic growth, but many see its long-term stability threatened by Islamic militancy and political uncertainty.