There are a number of elements that we can point to characterize this story as an example of "modernism". Modernism is generally associated with ideas of subjective reality (or subjectivism) and deals also with the influx of mechanization into every day human life.
In the story, the narrator has a different perspective on his injury and on war than his fellow injured soldiers. Among the other Italians, the Major holds a view that is different from the younger Italians. These differences in perspective are important.
The major - in losing his wife and suffering a physical injury - is never going to recover. His views on living have been challenged too much. The pictures of healthy legs on the walls of the physical therapy room are ironic to him. These same pictures are not ironic to the narrator who still has a good chance of recovery and some normalcy.
The narrator's personal narrative remains intact while the major's personal narrative is broken. There is more to be said about how the sense of reality is different (i.e. subjectively defined) for the characters in the story, though they share the same facility and the same modern therapy machines.
One point that brings the shifting sense of subjective reality to light is the major's sudden shift in attitude. After his wife dies, the major calls the narrator ‘‘a stupid impossible disgrace," where previously he had treated the narrator with cordial respect.
Suddenly the major's view of the narrator has changed. His sense of the world has been changed. This reinforces the psychological notion common in modernism that the mind defines the "real" according to its own fancies, pressures, idiosyncrasies, etc.
The machines themselves offer another connection to the movement and interests of modernism. The men in the story are supposed to put their faith in these therapy machines, hoping that the machines will heal their wounded human parts.
No machine can heal the major's wounded emotional being, however, and even the narrator has trouble believing that the machines can make his injured leg look like the healthy legs in the photographs hung around the therapy room.
Mechanization, here, represents a burgeoning expectation that problems can be removed through "modern ingenuity". The story casts doubt on this prospect, a doubt that is fully consistent with modernism.
The modernist writers, almost as a rule, feared the new technology... (eNotes)
We might suggest also that the major, in his rather mechanical initial attitudes regarding dignity and self-carriage, is symbolized by the machines that fail him. His rigorous attitude cannot save him from heartbreak. The machines cannot heal his damaged psyche when he loses his wife.
...having lost his innocent belief that loss can be minimized through discipline and precision, what the major sees out that window is life’s vast emptiness. (eNotes)