White Cavalry hymnals

I was just looking at the hymns of William W. Phelps, whom I think of as an LDS hymn-writer (see Wikipedia for a biography of Phelps detailing his rathered checkered history as an early Mormon leader), and noticed that one of his most widely printed hymns, one which appears in a number of apparently non-LDS hymnals, namely "The Pure Testimony Put/Poured Forth in the Spirit", most recently appeared (to the best of the database's knowledge) in the second (1936) and third (1951) editions of something called Warrior Songs for the White Cavalry, published in Boston by "The Kingdom". (The first edition, published in 1897 by a Bible school in Maine, does not contain the hymn.) At first glance I thought it must be some sort of Klan publication (though Boston is an odd place for that), but on googling I find that "White Cavalry" is associated with a purported miracle in the First Battle of Ypres during World War I, too recent to have inspired an 1897 publication, and that the term apparently has scriptural origins. Can anybody tell me any more about these hymnals, the sect they came from, or the particular eschatological underpinnings of the image? I'm quite curious.