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Thousands of American troops are expected to arrive in Haiti today. The security that their presence will provide is urgently needed, with reports of law and order fraying in the capital Port-au-Prince. The efforts of US soldiers should be invaluable in the relief effort too. Army medical teams can help the injured, and troops can distribute food.

But the docking of the carriers USS Carl Vinson and USS Bataan in Haitian waters will also be a symbolic moment in the relations between these two nations. America has a special responsibility towards this poor and now utterly stricken Caribbean neighbour.

From the earliest days of both republics, Washington has regarded Haiti as part of its sphere of influence. The human ties go deep. The US has an...