No, they would have warning, obviously. And they are likely at very high alert. The systems and command and control are meant to be quick and survivable for exactly that reason. But this is a unique and untested situation. There’s likely to be hesistation, screw up, and failure. American subs can launch on a depressed trajectory and hit Moscow within a few minutes. A few nukes going off at high altitude can disable almost all electrical devices in the whole country and there might only be 2-3 minutes warning on such an attack. The military communications are designed to survive this scenario, but it’s untested in the real world and at least some communications will be lost, adding to the confusion.
There will be a variety of human factors that can go wrong here. People can be in denial or disbelief. The right people needed to launch may be unavailable or hard to reach, especially in a rigid autocratic society. Crumbling Soviet infrastructure has a lot of potential failure points. It is likely that their dead hand/perimeter automatic launch system has never actually been activated because it’s a bit crazy and it’s more of a threat than an actual weapon anyone wants to use.
The US Navy is quite good at tracking Russian subs and there’s a good chance we could kill all of their boomers right before our strike. Russia’s nuclear air forces are rarely on high alert or that big a concern and will likely be destroyed on the ground. The strategic rocket forces are the real threat in this scenario, and there’s a decent chance you can pin down a significant fraction of their launch capabilities via direct attack.
The poster I was responding to suggests there’s no rush to nuke Russia because we can always get them with a second strike. It’s true that our second strike is likely to be more effective than theirs, because our infrastructure is more effective and we’re much better at hiding our boomers from them than they are from us. But in this scenario, you’re letting them have all the advantage of being the attackers. Having a plan, coordination, having all their forces ready and with orders. Not being the ones being confused, attacked, reacting.
Compare the situations with the subs alone - in one scenario, a US attack sub tracking a Russian missile sub sinks it a minute before the general attack starts. In the second scenario, with Russia issuing the first strike, the US attack sub maybe starts to suspect the Russian sub is setting up for launch, but it may not know and may not have orders to commit an act of war to sink it until after the missiles start being launched. That hesitation is likely enough to let a few birds fly.
In a US first strike scenario, if they get lucky/have a good plan, time their attack to be the worst time for Russian leadership, bring our best strategies to bear, it’s likely we destroy most of the Russian nuclear forces. In a Russia first strike scenario, they are likely to get more than half of their nuclear forces off. Saying “we shouldn’t first strike because we always have the option to second strike” vastly understates the difference between those two scenarios.