Complex sentence: The relief force did not know that 2,800 destitute Chinese Christians had taken refuge in the Legation Quarter with the foreigners, nor did it know that three miles distant from the Legations a second siege was in progress.
Split into simpler sentences: The relief force did not know that 2,800 destitute Chinese Christians had taken refuge in the Legation Quarter with the foreigners. Nor did it know that three miles distant from the Legations a second siege was in progress.