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With last Wednesday's decision to scrap plans for a promised missile defense system in Poland and the Czech Republic, President Obama put the finishing touches on a new and dangerous entity: post-allied America. With his declaration a week later before the UN General Assembly that "alignments of nations rooted in the cleavages of a long gone Cold War" no longer apply, he justified his creation. "As a string of headlines from Central and Eastern European capitals makes plain, the U.S.'s most reliable democratic partners see the administration's decision for what it was: a historic shift in America's priorities. Adversaries' wishes now enjoy equal baseline footing with the needs of friends. Whatever may tip Washington in this or that policy direction, a history of cooperation or shared...