Russia only needs to increase the air defenses of Syria. If them reach the areas controled by the Kurds the US will not be there in big amounts, because they only like to be on land where they control the air space, and where they can fly free.
Without strong air support and air coverage, there is not land presence of the US in the ground.
If Russia wants to abort the US backed opperation in Racqqa only need to help Syria stablishing a no fly zone for the US over the city. And for it there is not a need of MiG-31. Cheaper land warfare is enough.
In the refered to the air warfare, the best for Syria is to have L-39 in decent amounts. The US would dislike to waste expensive missiles on them. At same time is the cheapest and easiest to provide for Russia. Then it is clear what Syria will receive on air warfare.
Last edited by eehnie on Sun Apr 09, 2017 11:45 pm; edited 2 times in total