Guardian or King Philosopher?

Line 484c states, "those who look as if they're capable of guarding the laws and practices of cities should be established as guardians." At the end of class today we discussed how guardians and king philosophers are now one and the same. Before we were just speaking of guardians, but now we have gone in-depth into who they should be. I don't agree that kind philosophers should be the same as guardians, and I believe Socrates has contradicted himself as placing them as such. Earlier (I can't find the passage) he talked about how when you are given a duty you will focus on that duty and that alone so as to finish out that task to the best of your abilites. So, if king philosophers are to do just that and study, find knowledge, set the laws, interpret the laws, and teach; then isn't it true that they should not also fight for the safety of their city. They should not be the ones in battle because that will take their focus away from finding what is good and true. A conflict of interest will arise when one is out fighting the enemy from discovering what is good and true within their own mind. Also, what will the people do if all of their king philosophers and leaders are killed off because they're out fighting to keep the city safe. No one will be left that knows truth and solely searches for what is good. It seems to me that in doing both, they are creating a mess.