"They are not the enemy."
Translation:Ce ne sont pas les ennemis.
This is my question as well. The English sentence we're asked to translate is "they are not the enemy" because in English it is understood that "the enemy" can refer to a group of people (for instance, the country or countries being fought against in a war, or some sort of ideological enemy).
However, using the singular "l'ennemi" in French is wrong here, according to Duolingo. Can someone who speaks French definitively confirm that the singular cannot be used in this situation in the same way that it's used in English?
C'est/Ce sont is used when the noun has an article, such as les, des, un, une, etc. In your example, it is "les" ennemis. Check out the article: http://french.about.com/library/weekly/aa032500.htm