"But all that having been said, you can't, in a city of a million people like Karbala, or 5 million like Baghdad, you can't be in all places at all times"

This quote by John Abizaid is referring to the problem of preserving security in big cities. He is emphasizing that it is impossible to be in all places at all times, even in cities with large populations like Karbala and Baghdad. Abizaid is suggesting that it is difficult to keep an eye on and protect each and every single individual in a city of millions of people. He is implying that it is hard to guarantee the security of everybody in a large city, and that it is difficult to be everywhere at the same time. This quote is a tip that even with the very best security steps, it is impossible to guarantee the safety of everybody in a big city. Abizaid is stressing the significance of having a strong security system in location, however also acknowledging the limitations of such a system.