Before talking about his own experiences, Marlow wonders how a Roman force sent to expand their own empire would have reacted. This is the first indication of the power which the jungle has on him; perhaps unconsciously, he foreshadows some of his experiences by placing them in the hypothetical Roman force:
"They were men enough to face the darkness.
The fascination of the abomination -- you know. Imagine the growing regrets, the longing to escape, the powerless disgust, the surrender, the hate."
(Conrad, Heart of Darkness, eNotes eText)
Marlow connects his own fascination with and disgust of Africa to that Roman troop, sent in a time when communication over long distances took weeks, and without knowledge that their efforts had any positive results at all. However, he also mentions that they must have been "men enough to face the darkness," having the stalwart willpower to continue even in the face of adversity. This is a direct call-forward to Kurtz; although his actions were more brutal, he continued gathering ivory and sending it upriver. The Roman troop provides a sort of historical context for the reader, the indication that many empires of the past have tried and failed to conquer Africa, and that this will be the inevitible fate of European Imperialism.