A shooting war with Russia means the entire US East and West coasts are within range of sub-launched cruise missiles,
Which is why US attack subs are following the Russian boomers around...
Sure, but can we be sure we have found every one of them?
It isn't just the boomers that are a problem either - I imagine their attack subs can carry cruise missiles.
But, think about it. The only way a war could start and stay contained is if the US sends troops to the Ukraine, the Russians don't do anything to interfere with them arriving there, and the US troops just fight along a front with the Russians to push them back to the border. How likely is that? It would certainly be a bloody war - nobody fights that way since WWI.
The more likely scenario is that the US mounts invasions of Russia from somewhere OTHER than Ukraine, which would put Russia on the defensive. They would immediately have to pull out of Ukraine or they'll end up being encircled, or losing a city in the north like Moscow or St Petersburg. Since that would be all-out war the US would preemptively attack all Russian Naval assets, expecting them to retaliate with unrestricted conventional warfare otherwise. We are now talking about a global unrestricted conventional war between the US and Russia, that we hope stays conventional even though the US is fighting on Russian territory, and will likely capture quite a bit of it assuming they can get enough ground troops over there before the real shooting starts (there is a logistics challenge here as Russia is huge and the US isn't basing thousands of troops in Latvia or anything like that). If the gloves are off, then you also have to factor in things like special forces, sabotage, and so on. I'm not sure that Russia's only option for striking the US is submarines.
I'm sure this would make for a great Clancy novel, but nobody really wants to actually see this stuff happen. I think the US/EU would let Russia march right up to the border of Germany before they'd do something about it. Maybe they'd stop them at Poland this time or something. I doubt that they're ready to draw the line at Ukraine, or even the baltics. Everybody is just going to want to pretend that it isn't happening, or that Russia just wants a bit more buffer.