As pointed out above, it depends on the story, but in general I avoid backstory. I start the action in the present, with the situation and the characters formed as they are. That's how it is in real life when we encounter something new. We project upon the situation whatever it is that we bring with us in terms of experience and our emotional reaction. Whoever narrates can shade the story with hints that may help to explain what is going on (or went on), but in a short, that's not always necessary. So long as there is a beginning, middle and end, with a plot that moves forward, that's what counts. Just avoid creating a vignette where nothing happens (unless that's your goal and that's what the story editors are looking for, which is not very often).