There is a lot Russia can do to retaliate against the US if they illegally attack Syria ever again.
Putin can bring about the fall of the regime in Ukraine. He can also support insurgency against the ruling regimes in Latvia, Lithuania and Estonia and replace those regimes with pro- Moscow regimes or else occupy those countries just like the US is illegally occupying parts of Syria and previously Iraq.
Russia has interests in those countries and its slavic populations want to be freed from western EU and US influence and interference. Who can blame Mr Putin if he ousts regimes in the Ukraine and Baltic in revenge for the US meddling in Syria???
It's none of Americas bloody business, it's time the US butted out of the Middle East after the decades of damage they've done. There would not be ISIS had Iraq not been invaded illegally or civil war in Libya had the west not helped murder Gaddafi.
TBH the world is fed up of America attacking sovereign nations in the Middle East and leaving behind d chaos, death war, and misery. ISIS only exists because of the US, Turkey, Britain and Israel supplying weapons to terrorists in Syria.
As for Boris Johnson asking Russia to quit supporting Assad, well Putin and most of the sensible world think Boris Johnson is a grade 1 idiot.