His Plan A having flopped ignominiously, assume Putin is already moving on to Plan B, which could well be to encircle and starve the defenders of Kyiv. The historical analogies are obvious: Paris in 1870; Leningrad in 1941; and, of course, Berlin in 1948. How would we respond?
We, and the rest of the world, would not intervene to prevent an assault on the city, but we would not let its defenders starve. We would organize an updated version of the Berlin Airlift, using planes from neutral countries in an effort to avoid a military confrontation. Technically, it can be done. But what if Putin, unlike Stalin, shoots down the planes?
The US would have to wipe out Putin’s anti-aircraft capabilities. Americans would have to kill Russians. At that point, anything, including nuclear war, is a possibility. We have moved from Berlin in 1948 to the Cuba Missile Crisis.