A portion of an exhibit is shown in the museum area at the George W. Bush Presidential Library and Museum in Dallas on April 16. The George W. Bush Presidential Center, which includes the library, museum and policy institute, will be dedicated Thursday at Southern Methodist University in Dallas. Benny Snyder/AP hide caption

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A Spanish soldier aiming an arquebus in the New World, late 1500s. Hand-colored 19th-century woodcut reproduction of an earlier illustration. North Wind Picture Archives/AP hide caption

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President Woodrow Wilson meets with his first Cabinet, circa 1912. Should Cabinets have a more central role in a president's decision making? Hulton Archive/Getty Images hide caption

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Black smoke rises from the chimney of the Sistine Chapel on April 18, 2005. Black smoke signaled that the cardinals sequestered inside had failed to elect a new pope, after the death of Pope John Paul II. Alessandra Tarantino/AP hide caption

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