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Which is the greater folly: To fancy that war offers an easy solution to vexing problems, or, knowing otherwise, to opt for war anyway?

In the wake of 9/11, American statecraft emphasized the first approach: President George W. Bush embarked on a "global war" to eliminate violent jihadism. President Obama now seems intent on pursuing the second approach: Through military escalation in Afghanistan, he seeks to "finish the job" that Bush began there, then all but abandoned.

Through war, Bush set out to transform the greater Middle East. Despite immense expenditures of blood and treasure, that effort failed. In choosing Obama rather than John McCain to succeed Bush, the American people acknowledged that failure as definitive. Obama's election was to mark a new...