CaesarV[" ", n_] := " "
CaesarV[x_String, n_] := 
      FromCharacterCode[Mod[ToCharacterCode[x] + n - 65, 26] + 65] /; 
        Length[ToCharacterCode[x]] === 1
CaesarV[x_String, n_] := 
      Module[{length, first, rest}, 
          length = StringLength[x]; 
first = 
            StringTake[x, 1]; 
rest = 
            StringDrop[x, 1]; 
StringJoin[
            CaesarV[first, n], CaesarV[rest, n]]] /; 
        Length[ToCharacterCode[x]] > 1
CaesarE[x_String, n_] := CaesarV[x, -n]

test = "QXIIT TCIOXUUTGC"

Do[Print[CaesarE[test, n]], {n, 1, 26}]

CaesarE[test, 15]