From multiple sources available, i am of the opinion that the following things took place
1) On MBS's insistence, Trump started beating war drums with the bonus of deflecting attention from domestic situation.
2) Macron and May, couldn't resist the temptation of yet another practical military testing field.
3) The chemical attack was in all likely hood a drama, it makes no tactical advantage for Assad to use such weapons when he is already close to victory using conventional methods.
4) As such UN was bypassed, in order to start a war.
5) Russia, most probably, using back channels warned of severe consequences. Most probably some sane US generals would have also warned Trump of the Russian retaliation.
6) Trump needed face saving, Russia needed US to blow off some steam, and thus today happened.
7) Today was not at all a test of Russian S-400's effectiveness.
8) Russian promises. The value of Russian promises has gone down, they shouldn't have made tall claims of securing Syrian skies and retaliating the source of missiles etc as well. They could have said "We are trying to deter a full fledged war using all means available" or something on these lines. Keep it vague.
9) Imagine if Russia/China attacked taiwan/ south korea etc using missiles. Would US have said similar things that we destroyed 70% missiles, lets dance? So Russian defence umbrella has proven to be a bunch of big lies. It will hurt Russian power projection.
10) In the heat of the moment, what if few missiles were fired towards cyprus? Russian prestige would have shot the skies!
11) US, UK and french have wasted their efforts. They are the overall losers in the Syrian game of chess. These stupid displays of military power will not change the ground reality.
12) Assad has practically won. The finishing game is going on. The question is not if but when will entire Syria be under Assad rule.
13) Qatar and KSA can kiss goodbye to their dream of pipelines connecting arabian peninsula to europe.
The end
P.S.:- I am no supporter of Assad, but what i can say for sure is before western backed rebels started an armed civil war, Syria was much peaceful. If syrians want change, the change should come from within without any external power arming the opposition/al qaeeda etc. Furthermore, Assad from a constitutional point of view, has to leave his office permanently by 2021 anyways.