What additional foolishness does Tom insist on that creates great danger for them, especially for Jim, and brings a lot of armed men to the Phelps’? CH 39-end

Tom writes a letter pretending to be a member of a gang of robbers who are planning to steal Jim from the family, he warns them that the gang will be coming late at night from the north to get Jim. The family is terribly frightened and does not know what to do. The letter has a strong effect, and over fifteen armed farmers are sitting in the house waiting for the robbers to come during the night of the escape.