My ST-TOS fan theory:
In an alternate universe (that we would come to call the Mirror Universe) after having traveled back in time via a time portal, Kirk or perhaps even McCoy saved Edith Keeler from being killed by a truck (in "City on the Edge of Forever").
We know from that episode that if Keeler did not die from the accident then, through a series of events, the Nazis would win WWII.
So in this alternate universe she was saved, the Nazis won, and their fascistic world government along with the Axis powers eventually led to the tyrannical & fascistic "Terran Empire" that we saw in the episode titled "Mirror Mirror" (and again on Enterprise in "A Mirror Darkly I & II").
In an alternate universe (that we would come to call the Mirror Universe) after having traveled back in time via a time portal, Kirk or perhaps even McCoy saved Edith Keeler from being killed by a truck (in "City on the Edge of Forever").
We know from that episode that if Keeler did not die from the accident then, through a series of events, the Nazis would win WWII.
So in this alternate universe she was saved, the Nazis won, and their fascistic world government along with the Axis powers eventually led to the tyrannical & fascistic "Terran Empire" that we saw in the episode titled "Mirror Mirror" (and again on Enterprise in "A Mirror Darkly I & II").
Those are WWII German solders in that image.