Earlier quoted context omitted.
You are an editor on CNN. A person from the US has been elected as the Pope, you have to come up with a title for this news piece. You have these two options: A) First American Pope elected ... B) First US Pope elected ... A is ambiguous because "American" means a country for 4% of the world and a continent for 96% of the world. Also, the pope that just died happened to be from Argentina, and also happened to be the…
But “US” is not an adjective? It’d be like saying “First Brazil Pope elected”
"US Economy"
"US Job Market"
"US Military"
"US Policy"
And many other examples ...
But now that I read about it, when you use it as adjective you have to write as "U.S.".
If you want to throw the whole argument to the trash because it's missing two dots, well ... up to you.