What is the origin of the term French Kiss?

One theory is associated with France being known as the city of love, with the Eiffel Tower and Paris considered the ultimate symbols of romance. Another theory is that French Kiss was a derogatory term used during war: it was said that the French would rather make love with their faces than fight. The term dates back to at least the 1820s and became popular again during the 1920s when the French were believed to be preoccupied then with passion and love.