Just to drop my two-penno'th into the fountain:

During the war (2nd World/Great Patriotic anything save WWII [ugh]) the term "Yank" or "Yankee" was used all the time purely as a slang term for someone from the USA. There were no specific connotations to it and the term would be used just as much if we were praising your great nation or abusing it.
(This was, of course, in total ignorance that it was a term only appplicable to those ffrom the Union, rather than from the Confedracy. "Gone with the Wind" was probably the first step in our education on such matters!)

In those days, we could also refer to someone from north of the English border as a "Jock" and someone from west of the ditto as a "Taff" without rousing any ire. It is only over recent years that these words have been seen over here to hold any sort of pejorative connotation. Just about everyone had some sort of nick-name, and you only felt insulted if your friends and work-mates didn't care enough about you to invent one for you.