With the help of Harvard's Widener Library, which has *two* copies of _Rhymes of the Rookies_, I looked up the earliest printed source of the Marines' Hymn. Sure enough, it has the line in its diplomatic form, "They will find the streets are guarded". It is clear that "They will find that country occupied" is spurious. My guess is that it was made up, pretty early on, by overeager Marines.