Complex sentence: At the start of the war in 1939, there were 78,000 refugees in Britain who were considered "enemy aliens," a category that included all peoples living in the UK who were from Germany, Austria or Italy.
Split into simpler sentences: At the start of the war in 1939, there were 78,000 refugees in Britain who were termed "enemy aliens". "Enemy aliens" was the term used to describe all peoples living in the UK who were from Germany, Austria or Italy.