With each humanitarian crisis in Ukraine, the US comes that much closer to intervening in a direct way. Neither Ukraine nor Russia will capitulate, they're too evenly matched conventionally. So there will be many, many of these events happening in the future.

Already the leadership of certain NATO countries are calling for a no fly zone, and transfer of various weapons systems. It's a simple exercise to ask yourself what humanitarian disaster would be so egregious that the US would be forced to act because if it did not NATO itself would fracture, with newly-added former Soviet republics and eastern block countries acting on their own.

That event is not very far away. Mariupol is a disaster, but it's one that none of us can influence. We can, however, take steps now for the inevitable arrival of exchange of fire between US and Russian forces.

I'm happy to ratchet down that stand, but until something happens with clear de-escalatory potential, the base case needs to be direct US-Russia confrontation and likely nuclear escalation.

Edit: the split in NATO could come quite quickly. A low-yield nuclear first strike by a cornered Russia into Ukraine would put the entire immediate vicinity (the Baltic states, Poland, etc.) on alert that they're next on the list. The pressure on local politicians to act first and receive reprimand from NATO later could be irresistible. The US's "ironclad" commitment to Article 5 does the rest.