It isn't Iraq especially in the sense that Iraq had no where to go. On the other hand Russia can get the ** out of Ukraine and then carry on (more or less) as before. Yes, their economy is totally doomed now because it depends on energy that they are going to have a struggle to sell - for 10 years at least, but they do have a deescalation option. Iraq did not. Ukraine does not.

> Isn't "sinking the Russian Black Sea fleet" directly striking Russia?

Under certain viewpoints it is the difference between attacking the US command in Iraq and striking troups in New York.

The black fleet is stationed on annexed land as of the view of most UN member states. An attack on troups stationed on foreign soil is not the same as an attack on troups stationed e.g. in the capital of an country. The black fleet are military targets on the soil/waters of an invaded country in an attack war. This is the status most foreign ministries would tell you the black fleet has currently.

The question now is: What are the consequences of doing nothing, should Russia deploy tactical nukes?

Most analysts would say there are bigger consequences to doing nothing than to doing something of limited scale unless your goal is normalizing nuke deployment (very bad idea).

So where would you retaliate instead?

Weren't any nukes onboard with Russian Black See fleet? I guess in their position, they wouldn't mind to take the whole world down with them by launching all nukes onboard?