This paper makes two arguments. The first is that Trajan deliberately orchestrated the dedication of his Column on 12 May, the anniversary of the dedication of the Temple of Mars Ultor, to coincide with the beginning of a new war against Parthia in
a.d. 113. The second is that although most modern commentators focus on the function of Mars Ultor as avenger of Caesar, the evidence of his actual invocation from the late first century b.c. through the third century a.d. more strongly supports another interpretation: as agent of vengeance against foreign enemies, and against Parthia/Persia in particular.