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In the affairs of states, lessons are often learned too late or too well. Faced with unexpected crises and unwelcome demands for prompt decision-making, governments think by analogy. And they are invariably keen to demonstrate that they have learned from their--or, more conveniently, their predecessors'--mistakes.

The last time a Democrat occupied the White House, an inherited humanitarian mission in Somalia turned to disaster in the alleys of Mogadishu. An important lesson had to be learned: Committing troops to African conflicts of little strategic import to the United States was no longer on the table. And so came to pass a genocide in Rwanda--during which there is no question that a "Somalia effect" darkened the considerations of every...