From what I've gathered, the French were very powerful during the 18th and 19th centuries, and we owe a debt of gratitude to them for the victory in our own Revolution. So, when/how did the "cowardly French" stereotype emerge?

hi! not discouraging further discussion here, but FYI there's a FAQ section on this topic

Reputation of the French military (origin of the "surrender monkey" cliché)

note that if you have follow-up questions you can post them here; just mention the user's name (e.g. /u/Searocksandtrees) so they may get an automatic notification