Yea get ready for world war three.
Don't get ahead of yourself.
World War III can't really exist the way you might be conceiving of it. This can never happen so long as thousands of ICBMs with nuclear warheads affixed exist. Putin even references this in his letter.
I don't even think this really reads as a threat of force from Russia.
It more looks like Russia is saying, "Look if you want the UN to hold sway--then you have to follow the rules, too."
Which basically means that if we circumvent the UN charter, Russia wouldn't feel wrong to do the same--and thus the UN would be rendered essentially useless. Again, he references this in the letter by discussing the League of Nations. This is exactly what happened to LoN. In its early days this is also what caused the failure of OPEC (since leverage has been installed, OPEC has become VERY powerful).
Putin isn't saying, look out--we'll bomb you. He's saying, look out--this will move us backwards.
All that said, I think even IF WWIII was on the horizon it wouldn't be a result of this situation. I think it's pretty clear at this point that Syria is going to hand over its chemical weapons and this chapter will come to a close. It doesn't make sense to strike when we've got a win-win-win situation.
Syria doesn't get bombed, they're happy. Russia gets to back their ally and stand resolute against the US "bully", they're happy.
We get Assad to give up his chemical arms, we're happy.
win-win-win.
I think the likelihood of a strike has gone to very near 0% at this point.
