At the start of the invasion Russia was claiming they finally had hypersonic nuclear missles. Meaning they could get nukes past defense systems. That is the reason the US was hesitant to let things escalate.

Since then we’ve found out that Russias hypersonic missles had some misfires and still didn’t have the ability to steer at hypersonic speeds if I remember right.

That plus the fact that Trump is months away from helping Russia changes the strategy.