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America’s closest allies in Asia are wasting no time establishing new economic and political alliances that can diminish the role of the U.S. in that region. Both Japan and South Korea are setting their own courses to an extent long considered beyond their reach. If you accept the popular premise of a “post–American era,” this is what it looks like when it arrives.

 Leadership changes in Japan and South Korea prompting new economic policies focused on deficit spending and demand stimulus—straight-ahead Keynesian economics. We are also likely to see foreign policies that ease tensions in the region. Not least, Seoul wants to revive something resembling the “sunshine policy” toward North Korea that Kim Dae-jung, the late South...