Let's say there is a flash call to the revelant combat commands who in turn contact the white house at 3 am.
It will take 10 minutes to chop the prez out of bed. Time enough to nuke DC.
Then it will be a fixed amount of hours as the assembly and orders go out. Provided the communications stay up undisturbed by the enemy. I fully expect Russia to try and take down our internet, both civilian and military in a shooting war. Even try to get at our water treatment plants, local power grids and satellite TV etc. Poof. Distract the DC leadership when 330 million Americans scream in the dark unheard.
Russia probably would have a Air Army dropped into Kiev and Kharkov and stormed Odessa by Lunch the next day. The rest of the heavy stuff comes through the borders.; It might take half a day in some areas but reserves go with the forces that broke out and is running free inside Ukraine. The rest tie down the enemy.
If NATO mobilizes then its no problem for Russia to start taking out NATO targets. Kallingrad up in the old Prussia would be a one hell of a hornets nest.
Biden does not have the mental capacity to take the shock when the shooting starts. And then accumulates a tidal wave of issues that increases by the hour. All of which demands attention.
While DC dithers, the American People demand something, anything and it would be days before we get moving. What no one understands is that if Russia does a lightning move Ukraine belongs to them at that point. It becomes a big time police problem for Russia.
IF we so much as attacked a Russian anything inside their National Borders or any of their units on the land, sea or air we could be seeing the end of the human race delivered by missile in 30 minutes. Our people and way of life will simply cease to exist. Same with Russia.
I dont worry about it too much. But Biden is not the person you want in a time like this. I can probably deal with it myself but I am not President for a number of very good reasons.