I've done a little reading on them--mainly through research in the transcripts of the hearings on the Rangers that the legislature carried out in 1920 about the events from 1915 forward. The hearings had several witnesses about their work in the Valley and elsewhere in trying to enforce the draft laws and prevent dodging service.
There's also a good book out on a German_American citizen from Seguin (if you Know Texas, you know we have a large German immigrant pop.) who was caught in the toils of the loyalty Rangers during the war. Can't think of the name off-hand.
One thing that is forgotten that it was not a good time to be a German-American during WWI. You may have heard of sauerkraut being renamed "liberty cabbage" and hamburgers "salisbury steak." German language publications, already plagued with declining circulation, were ordered to publish in both English and German so that the censors would know what they were writing about. Many folded as a result. Towns changed their names -- here in NJ, "German Valley" became "Middle Valley." In general, German-Americans were the object of suspicion, though I doubt that they whined about it, unlike modern-day Muslims in the US.
After WWI and the Bolshevik revolution, there was the "red scare" and the Palmer Raids, but that's the subject of another thread.