Since the end of World War II, our country has had three great presidents: Harry S. Truman, Dwight D. Eisenhower and Ronald Reagan. Their politics varied, but these giants stand in sharp contrast to our last three presidents, Bill Clinton, George W. Bush and now Barack Obama. The first two presided over gravely flawed presidencies; the third is on his way to outright failure. What makes these two presidential trios so different? A recent visit to the Truman Museum and Library in Independence, Mo., made me ask what made those great presidents great. The answer is character. The three greats...