What is the etymology of "french kiss"?

The french kiss is a kiss where the participants' tongues are used to touch the other participant tongue or lips. This is an expression I have heard since I'm little, but I am very curious about the possible origin of this expression. It seems that the expression is British, as there is a lot of expressions there using the word "french", such as French letters, to take the French leave or excuse my French.

Wikipedia seems to describe the etymology this way :

A French kiss is so-called because at the beginning of the 20th century, the French had a reputation for more adventurous and passionate sex practices.

But is that really all ? Is the expression that recent ?

French kiss is from 1918 according to The Routledge Dictionary of Modern American Slang and Unconventional English.

The following source cites its first usage in the WWI Private Lindner's Letters:

• While "French kissing" began appearing in conversation during the 19th century, it took off in the early 20th century when it first appeared in the World War I book Private Lindner's Letters: Censored and Uncensored of 1918. "I have decided to become a linguist," he wrote. "Being able to read French fluently and speak it wretchedly, and to speak German connectively but not to read it at all, I am taking up Luxembourg, which is a wonderful blend of the two, a sort of liaison between tongues. (Not to be confused with French kissing.)"

• In fact, if we're going to credit anyone, then it probably should be the British and American soldiers of World War I. Upon returning home from continental Europe, they showed their wives and girlfriends what they learned from the amorous country and called it a French kiss.

(www.popsugar.com)

As noted in Quora, the term French was used in a number of prurient expressions at the beginning of the 20th century:

• I believe the term originated in England at a time when adding the term "French" to something already salacious made it even more prurient. For example, syphilis was called the "French pox" by Englishmen and condoms were referred to as "French letters," for some reason. The "French kiss" is simply taking a relatively chaste kiss a step further to a deep, tongue-involved kiss and attaching further sensuality to it via the term "French." Of course, that sort of kiss has probably been around as long as there have been human beings, and certainly isn't the invention of the French.