Google-fu (and YouTube) have entirely failed me on this one, and yet I can't believe that it's so obscure that the material is simply unfindable. Here's the story:

In grade school in the late 1960s (US, Pacific Northwest), we were shown a series of social-studies/history films produced -- if memory serves -- by NBC (possibly the news division, possibly an educational sub-unit of the news division), covering a series of time periods from (I think) the Revolutionary War right up to the then-present.

These films, being aimed at grade-school viewers, included songs; I think most of the individual films used familiar popular and folk songs relative to the period or topic of the particular film. There was, however, at least one recurring song/lyric -- I don't think it was quite a theme song, in the sense of opening and closing every movie in the way of a television episode, but definitely repeating from one to the next and setting an overall arc for the film series. Here is the fragment of lyric that I remember, which I believe would have been the end of a verse and possibly part of the refrain:

"...built this land of opportunity;
A nation of immigrants, of hard-working immigrants,
Are we."

You can see, I expect, why this has been running through my head lately; does anyone else remember those films and the song in question, and if so, know where and how to track down lyrics if not an actual recording?