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America’s war in Afghanistan is winding down, but the U.S. must worry about conflict elsewhere.  Once viewed as inconceivable, war between China and Japan now looks possible, though thankfully still unlikely.  Tokyo should get serious about its own defense.

The U.S. used its power as occupier after World War II to impose a constitution on Japan which forbade possession of a military.  But America lost its enthusiasm for that arrangement early during the Cold War.  When Washington subsequently pushed Tokyo to rearm, the latter hid behind its constitution.

Japan’s neighbors also opposed a Japanese military revival, preferring to rely on America for defense.  Moreover, there were political points to be scored from attacking Tokyo....