RODBAR, Afghanistan -- At 10 a.m. Sunday, a dozen ragged schoolboys, who had been milling around this village's school after a meeting between Western soldiers and local elders, melted away, followed by their white-capped headmaster. Then, 30 seconds after the last boy carried away the blackboard, the ambush began. From crevices in the craggy hills to the north, Afghan insurgents fired machine guns and rocket-propelled grenades at a French Foreign Legion unit that had entered Rodbar earlier in the morning.

The ensuing firefight...