[flagged]
Of course, this would be very expensive to bring back their army from all around the world. They would lose all strategic positions they have, and would probably still suffer great loses... But they could (maybe) conquer that single country, or relevant parts of it.
Just, would it be worth?
https://www.oryxspioenkop.com/2022/02/attack-on-europe-docum...
I think that qualifies for a great big [Citation Needed].
Also, if they used that, there wouldn't be a county left to take over. Look where Chernobyl is, and its fallout zone, and ask how many other similar would happen as a result of using nukes to try to win.