First, the grammatical difference:
"Era" - Indicative past simple
"Fue" - Indicative imperfect
Sometimes, they are interchangeable. However, the past simple is usually used to say that comething happened once, at a single point in time, and want to emphasize it's over; while the imperfect reflects an ongoing action, or just a description of a past event, or a description of the circumstances. So:
"Era el hijo mayor." - When describing relationships, we usually use the imperfect, as a relationship almost never ends. And even when the person dies, we often talk about them during the times when they were still alive.
Fue una dictadura dura.
Era una dictadura dura.
Both are possible, but can have different uses. "Fue" emphasizes the dictatorship is over. "Era" describes the situation and , while other events were taking place, the dictatorship continued. You can also say "era" if the dictatorship changed it's nature (for example, from being "hard" to "permissive": "Era una dictadura dura. Luego se volvió un poco más permisiva.") but didn't end.