Johnny Reb Marines in Beirut?

And for the WTFO Picture of the Day, we have the U.S. Marines marching through Beirut in 1958 with ... a Confederate flag.

Sean and I are trying to figure out the meaning of this. I guess this would make sense if this was an old state flag, but a) I don't think it is and b) the USMC doesn't have National Guard units, so it doesn't make sense why they would be marching with a state flag anyway. Nope, this is the Confederate Battle Flag, alright. Anyway, this is just ... weird.