“OK, so how does one explain that the Russian S-400 Triumf air-defense system did not work as they warned the USA it would?”
How do we know they used it?
“But why? Why would the UK and France participate in a Wag the Dog scheme?”
Consider that they all had enough intel that they were only 60-70% certain Assad did it. Maybe someone else wanted to start a war between the powers to get him ousted. This was tried before in 2013. Consider how Russia has not responded.
RE: How do we know they used it?
Well, all those warnings by the Russkies that they will shoot down any missiles fired on Syria are baloney then.
See here for one:
https://nypost.com/2018/04/11/russia-warns-any-us-missiles-fired-at-syria-will-be-shot-down/