Till now Moscow was fine with Israel regularly bombing Syria struggling with al Qaeda and ISIS invasion. Till now, its anti-aircraft missiles have been deployed in Syria explicitly to defend the Russian bases only. Till few days ago Lavrov was still desperate to revive the deal with Kerry and was offering yet another 72-hour phony, unilateral "ceasefire." Today, official Moscow declared that its missiles would also protect the Syrians. This is happening in the midst of nuclear scaremongering, which appears to be a requested background for the last few weeks of the US presidential campaign in which Trump is now being openly attacked as a fascist, a Nazi, and a Hitler. Moreover, Putin's sudden balls and militant defiance is coming right after the last JIT report which is paving the way for a tribunal (already supported even by China) where the only accused and condemned is already framed to be Putin and his subordinates.
The Lavrov-Kerry deal, which was to give control over Syrian Air Force to the US, effectively banning it and breaking siege of a Qaeda-led force in eastern Aleppo, fell apart because US hubris and the perceived "target of opportunity" in the form of the besieged Syrian troops in Deir Ezzor by ISIS. The second likely explanation is that the US military deliberately hit the Syrian troops in order to kill and sabotage the deal by the same stroke. The deliberate attack on the Syrian troops in Deir Ezzor just two days before the de facto no fly zone for Syrian Air Force was to start forced the hand of the Syrian government on account of the justified, righteous wrath.
The surrender of Syria via the Lavrov-Kerry deal was to be the second best of such surrender following the previous failed deal, which was to "transit" President Assad out of power and deliver the government, the apparatus, and the army into the hands of the unelected "moderate" allies and minions of al Qaeda and give Syria a new US-written constitution.
For the West, selling Donbass by Moscow is not enough. It was never enough. Even Crimea will not be enough. Even Donbass together with Crimea. However, in arranging the surrender, the surrender and sell-out was supposed to be well timed and perfectly sequenced. First Syria, then Donbass (with an alternative of keeping Donbass pestering). However, in all this, three other managed or mismanaged processes have intervened: 1) implosion of the farce and lies painted over Putin, 2) implosion of the Russian economy that is now 50% less than the economy of Italy and less than the economy of Spain and still shrinking, 3) and the careful timing of the release of the new JIT MH-17 report, which was planned to come right after after the selling-out of Syria and the closely to-be coordinated surrender of Donbass.
Instead of working on the steady and secure improvement of Russia, its economy, society, and spirit on a day-to-day basis, Russian oligarchy preferred the regime of mafia rackets and Potemkin villages. Now after squandering 25 (or 16) years and the country's degradation, Russian comprador oligarchy is latching unto a straw of dramatic gestures, (and as before) wishful thinking and an idea that some sudden trick or magic may work or may be at least faked again.