The heartland of South Asia is the Indo-Gangetic Plain, a large and fertile region based around the Indus and Ganges river systems, as well as the Bramaputra in Bangladesh and eastern India.
The center of gravity of South Asia is the balance of power between Pakistan and India. Historically, India has struggled to assert itself within the subcontinent because of its complex internal divisions.
The future of Afghanistan matters to the degree it could weaken Pakistan, thereby tipping the balance of power in India’s favor.
There is an intense rivalry between China and India, but the geography of the region, including the existence of buffer states such as Nepal and Bhutan, typically prevents them from coming to blows.