Francois Aubert, Photograph of the execution of Maximilian I of Mexico (right),
Miramón (center) and Mejía (left) (19 June 1867)
Notice the nopal above/behind Maximilian, that was definitely not a coincidence. That might be the reason why he didn't stand in the middle of the group. The erected nopal as a symbol of Mexico versus the defeated foreign intruder... bla bla bla...
Edouard Manet, L'exécution de Maximilien (1867)