I don't know the exact location of the flag but I thought it was in reference to Gettysburg being the northernmost point the south reached during the war.
https://en.wikipedia.org/wiki/High-water_mark_of_the_Confederacy
Which means Q is telling us that the [enemy within] has gotten as far as they will be allowed to go.
From this point on, they will be pushed back.
At least that's what I took from it.