I feel that this is an ambiguous statement and could be passé composé (as the act of transition) or l'imparfait (as a state of being). As in "My mother was Oriental, but my father came from Europe" vs. "They came from Europe to go to the funeral". Compare: "Once upon a time, a king lived in his castle."
David, the ambiguity is in the English, not in the French.
As Jim notes, the context and meaning to be conveyed are important in choosing between passé composé and imparfait, and the tenses convey different meanings, which are readily understood by native French speakers.
While ‘they came from Europe’ on its own doesn’t give the context - in French the context is more easily deduced from the tense used by the speaker or writer.
The 2 other English sentences in your question do provide context.
The following link is a good practical explanation of how the French tenses are used. The little clips at the end are worth watching too.
What is being described here is a "background" situation, the action is not complete. Simply a statement of what happened historically, to set the scene of the story.
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