The line isn't clear, because there is no line. These lines you keep bringing up are just gamesmanship. Nothing changes because any of them are crossed. The war was fully escalated when they invaded. Ukraine has every right to attack targets in Russia. Russia and everyone else is just posturing to hopefully extract advantages. Everybody is trying to figure out what they can get away with that doesn't negatively impact them. When Trump won the situation changed for the current administration. Do you believe Russia wouldn't use nukes if it would strengthen Russia? Do you believe Europe and the US wouldn't have immediately shut down the invasion if Russia wasn't a nuclear power.

You don't think use of nukes constitutes any kind of 'line'?