It depends on your definition of the terms "start" and "nuclear war". From the Russian point of view, it was NATO-backed Ukraine, who attacked Russian allies - DPR and LPR. Second, de jure they don't have a term 'nuclear war' in their military doctrine. They have the terms 'local war' (which is fighted on the territory of one or few states), 'regional war' which is fought by the countries of one region (for example - Europe), in which they use both non-nuclear and nuclear weapon, and 'large scale war', fought by countries from more than one region (for example from Europe, Asia, North America), in which they are going to use literally everything.
So, when they decide that European facilitation is, in fact, participation, they will declare regional war and attack their enemies (first of all - nuclear weapon of the UK and France) with their nuclear weapons.
And if there is a choice between conventional defeat and nuclear victory - they definitely will choose latter.
Anyway, there is a plenty of pretty sane persons in Russia who think, that they can use a nuclear war for achieving their political goals ("Let's transform the World War into World Revolution! ")
They can win this Ukrainian conflict, and I'm sure that Russia and the USA can find the mutually acceptable solution for this European security crisis.