Like an unwelcome specter from an unhappy past, the ominous word "quagmire" has begun to haunt conversations among government officials and students of foreign policy, both here and abroad. Could Afghanistan become another Vietnam? Is the United States facing another stalemate on the other side of the world? Premature the questions may be, three weeks after the fighting began. Unreasonable they are not, given the scars scoured into the national psyche by defeat in Southeast Asia. For all the differences between the two conflicts, and there are many, echoes of Vietnam are unavoidable. Today, for example, Defense Secretary Donald H....