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Fifty years ago, a metaphor became concrete. Beginning on Aug. 13, 1961, along West Berlin’s 27-mile border, the Iron Curtain became tangible in a wall of precast slabs of concrete. It came down 22 years ago, but the story of how it rose, as told in Frederick Kempe’s book “Berlin 1961,” compels an unflattering assessment of John Kennedy. His serial blunders that year made it the most incompetent first year of any presidency.