In short, Nuclear War.
The invention of fission weapons put a real spin on the balance of power issue. Especially since now the cat is out of the bag anyone can build a nuclear bomb. Lithuania, Ukraine and Sweden can become nuclear armed nations within months of the directive being given. Germany too, before they abandoned their reactors. So 2 years for them now.
Greece, Turkey, Romania, and Poland can also become nuclear powers within a year or two, given sufficient motivation. Keep in mind South Africa was nuclear capable for a few years before it got sanctioned to death.
And this is why the US cares a great deal, because nukes have changed the cost benefit analysis, and game theory is very clear. The more players their are, the greater the likelihood for Armageddon. The US, HAS to guarantee the stability of Europe at this point. The alternative is fucking Lithuania developing nukes and aiming them at Moscow and Kaliningrad.
This more then anything else I think is why official Washington is furious and scared. They thought Putin understood this. Since it is clear he does not, the question is now what game he is playing and what the rules are.