Um, you have taken him a bit too literally. He did not mean that Roman legionnaires (or any of the other conquerors he mentioned) actually patroled Baghdad streets, but they did patrol the streets of conquered lands in the same way we now patrol the streets of Baghdad. It is not necessary to term someone an "idiot" because he is making a graceful comparison.
Baghdad is said to have been founded in A.D. 762, so it didn't exist yet in the days of Caesar or Trajan. Caesar never got that far east, but Trajan's last campaign was in Mesopotamia shortly before his death.
We are on patrol today in Iraq. Men and women of the United States armed forces in armored vehicles patrol the streets of Baghdad. They pass in the way of so many who have come before them: the Egyptian charioteers of Ramses II, the Macedonian phalanx of Alexander the Great, the Roman legionnaires of Caesar and Trajan, the Crusaders of Richard the Lion-Hearted, the legionnaires of Napoleon, the Camel Corps of Lawrence of Arabia.
All of these have come through the Middle East
Capriole: Um, you have taken him a bit too literally
Reread the above. No I have not. Dr Fears statement is hysterically inaccurate as I pointed out.