This is a fictionalized account of a raid into Mexico led by Colonel Ranald MacKenzie in 1873 - completely illegal, but with the tacit support of President Grant. It was aimed at destroying Mexican renegades and Indians who would raid into Texas and then flee across the Rio Grande into Mexico, where the US Cavalry couldn't follow. MacKenzie planned and executed a precision raid that put an end to the renegades. The novel was later the vague basis for the TV series "MacKenzie's Raiders" (1958-59).
It's an interesting slice of history, but the novel is very uneven. There are certainly some nice touches, but it's clear that the writer isn't very disciplined, as the points of view switch, sometimes in mid-sentence. Parts are good, parts are embarrassingly poor. On the whole, though, it's a decent read.