This painting by Édouard Manet is a political statement against modern barbarism, and especially against Napoleon III who at first imposed Maximilian as the emporor of mexico, but after the resistance he withdrew his forces out of Mexico.
Manet drew several versions of that painting. The second picture is another version of the same execution by Manet. It is very interesting to see how he decided to depict the executioners with more formal clothes, and in formal rows, making it look more calculated.
Choosing which side to take the painting also has a say in the painting. The third picture is of a print of the same execution done by Mejia Miramon, and was also published in 1867.
The forth example is Goya's 'The Third of May 1808', and it is a political statement regarding the Spanish resistance against Napoleon's great conquest.
It was very historically wise of Manet to use the same concept of sides (unlike the print on example 3). although there are a lot of similarities, it is very noticeable that Goya's painting is much more horrific, as if Manet was trying to keep Emperor Maximilian's dignity (and even the sense of a fatherly figure in the second painting).
The last picture is of a painting by Picasso, created much much later, in 1951, 'Massacre in Korea', where it is very clear that Picasso took the same concept and placed it in the fields of Korea with children and women on one side and robotic soldiers on the other.