Pakistan, Sept. 11 -- In Homer's Odyssey, one of Odysseus's most terrifying passages comes when his ship must sail through a narrow strait between two seemingly unavoidable dangers. On one side waits Scylla, the many-headed monster ready to seize sailors from the deck; on the other churns Charybdis, a monstrous whirlpool capable of swallowing the ship itself. There is no safe route and no possibility of remaining still. Odysseus has to navigate between the two.

More than two millennia later, the image offers a striking metaphor for India and Pakistan. Today, however, the most dangerous whirlpool in their relationship may not be found on a battlefield. It is flowing through the rivers of the Indus basin.

The latest turn came on August 31...