Union is a pretty poor translation for 'forening'! Club or association probably come closer to a proper translation.
Technically, the verb "at forene" means "to unite" so going from that, a forening would be a union (among other things, of course), just like De Forenede Stater/the United States, and genforening/reunion.
Så lige at du er dansk. "Association" er allerede et af de accepterede svar for union, men mange tak for tippet!
So do you use forening for the USSR? "Den sovietiske forening"?
No, we actually use "union" to describe the USSR - Sovjet-unionen. It's also used in Den Europæiske Union - the European Union. If the United States were called "The American Union" it would probably be called "Den Amerikanske Union" in Danish, since "de Forenede Stater" is a direct translation of the United States.
Oh wow thanks for your insightful answer! So if I may bother you once more, how do you say the UN then? "De forenede nationer"?
Yikes, this was not fun to get for a listening exercise