If Russia shoots US troops that are simply defending themselves while trying to keep a Russian army from advancing into Eastern Ukraine, the US will probably to go to war scale war with Russia.  It's that simple.  It doesn't matter what Russia claims because it is the US population and political class that makes that call, not international opinion.

This is how this works:

In 1969 the British sent troops into Northern Ireland to help protect Catholics who were being burnt out of their houses and attacked by protestants mobs with the shadowy support of the protestant police force and B Special auxiliary police  force.

They were welcomed by the overwhelming majority of the Catholic citizenry.

However a small minority of catholic nationalists/republicans didn't want British troops on their turf and started some semi covert attacks on the troops from the cover of local knowledge. The troops responded unskillfully and started inadvertently killing innocent Catholics in response.  Within months the entire Catholic community was turned against the troops and turned a blind eye when a full scale guerrilla war against the troops gradually emerged.

It won't take so long in the Eastern Ukraine, because unlike Northern Ireland (where the troops  gained the support of the majority protestant community) US troops have basically to legitmate reason for being in Uktraine.

If, as you say, the US political classes will make the call, at some point, to go to war with Russia in response, they are simply setting themselves up for a bigger disaster than Iraq, Afghanistan and Vietnam combined.  The US will lose its dominant position - economically and politically, in the world regardless of military outcomes in Ukraine.

And don't make the mistake of thinking Russia won't go full scale thermonuclear in response to (say) the US using Neutron or other battlefield nuclear weapons in the Ukraine.  (You use nuclear weapons on our turf, don't think we won't do so on yours).

It never ceases to amaze me that deapite Korea, Vietnam, Afghanistan and Iraq - all wars the US essentially lost -  the "US political classes" still think they can win such wars such is their imperial arrogance. And Russia is not Vietnam. You will be creating an enemy many orders of magnitude stronger.

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