They were on the front lines of a presidential security operation that could not have ended worse.

So it’s natural that some of the Dallas police officers on duty on Nov. 22, 1963, still wonder if things could have been different. They’re still bothered, after 50 years, by questions from outsiders, especially the conspiracy buffs. The pain lingers like the trauma of war.

“I have been at the bedside of numerous officers who have passed away and who were there at the time of the assassination,” said Paul G. McCaghren, a retired assistant police chief. “I have heard a number of them say, ‘I have never had such a horrible day in my entire life.’”

Yet, there are reasons to be proud of the police work done in the tumultuous hours and days after the assassination of President John F. Kennedy....