How do you do the possessive on “United States”? United States’ or United States’s?
DW
In ancient times, it would have been United States’s.
Then, the standard changed to United States’.
Now, a lot of people have gone back to United States’s.
Do what you please.
Then there are terms like United States Army, where no possessive is indicated. My guess is that where the world following United States is not capitalized, I would use “United States’s land”, as United States is a singular entity, a nation, and not merely a collection of states.
I’d re-word the sentence, really. The _____ of the United States…
Never once in my years in editing social studies materials and writing US history books have I seen either possessive used. It’s just not worth it.
Carol
PS But Doug makes a good point. Those are adjectival rather than strictly possessive, I think?? You could say United States territories as you would say French territories, rather than France’s territories.
Yes, that’s an elegant solution!
Dale
Or, failing all else, we could move to a country where the possessive is clearly defined (as in Carol’s France/French example).
That does it. I’m moving to Chad.
Dale
I hope you don’t get poked or punched in Chad 
MarkB
I confess I would just say ‘The United States policy’ say,
as nobody on earth will get confused.
It means the policy of the United States.
But that only works for the spoken word, where the difference in sound between States, State’s or States’ would be indistiguishable. In written form, I would go with States’, as it is a plural possessive, i.e. there are more than one State, and contrary to what some may think, they all act very independently. 
djm
Oh, how people will carry on when they don’t know the answer! Tsk tsk.
Dale, this follows the pronunciation. It’s very simple.
Write it as “the United States’ whatever.” It is pronounced as “states.”
United States’s would be pronounced “United States-ez.” You never hear that, so you would not want to write 's. You see? Very simple.
In contrast, when writing about the Jones family, you would write “the Jones’s car,” because you would pronounce it “Jones-ez car,” not “jonz car.” Unless you did want to pronounce it jonz, of course. But, I’ll bet that even if you think you pronounce it jonz, if you say it out loud you’ll hear yourself trying to tack on an “ez.”
What’s wrong with Jonz? I say Jonz. I like Jonz. I’m gonna keep up with it.
djm
It might be different in Canada. I think it best for you to stick with whatever you’re used to.
Might just be a Zed thing.
Go Canucks!
This is correct. Standard usage in the U.S. is to just add the apostrophe.
Redwolf
Yeah, that’s what the U of C Style Manual says. It also says that when you use the term as an adjective you should abbreviate it, as in U.S. history. The Government Printing Office, not that that’s any great authority, acknowledges this as an exception and seems to agree with jim stone.
I’d still reword.
Carol
i would write: states’
“states” is plural. You wouldn’t write “dogs’s” or “cats’s” would you? Ergo, United States’.
Or just get around the whole problem by “United States of America’s”
Maybe we should rename our country to make it less complicated.
You could always say, “America’s.”