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Back in January when he took office, Barack Obama had amassed a very considerable pile of chips. Events since then have appreciably reduced that stack. Should he wager what remains on Afghanistan? That's the issue the president now faces.

The first true foreign policy test of the Obama presidency has arrived, although not in the form of a crisis coming out of nowhere announced by a jangling telephone at 3 a.m. Instead, a steady drip-drip of accumulating evidence warns that Afghanistan is coming apart.

Unlike his predecessor, Obama has by no means consigned Afghanistan to the back burner. Since becoming president, he has declared the war there both necessary and winnable. He has ordered an increase to the U.S. troop commitment. He has installed a new commander....