This book (originally published in 1973) is Number Five of a series of six Phillip Hazard war tales (by this publisher -- there are actually eight "Hazard" books) and stands wholly on its own as a novel. Author V. A. Stuart claims that the events and all but a few names are historically correct and has included a bibliography at the conclusion of the work. In any case this is really a dandy adventure tale set in an era and location around which woefully little has been written: The 1850s Crimean War and specifically in the Caucasus Mountains, east of the action on the Crimean Peninsula.
In this entry the English Commander Phillip Hazard is ordered to round up some foot soldiers from among the Circassian Tribes of the Caucasus Mountains and to pair them up with their off-and-on allies, the Turks. The top commander of the French allies has gotten weak-kneed and recalled his soldiers just prior to an assault on a coastal town, held by the Russians, on the eastern edge of the Crimean Peninsula -- thus the abrupt need for more troops.
As Hazard doesn't have much time, just a couple of weeks to round up thousands of trained men, he also has to locate the man who can make all this happen - this is the Circassian tribal leader who is concomitantly preparing an ambush on a Russian supply caravan far from the coast. So Hazard leaves his ship with a few dependable men and embarks on the dangerous mountain journey to retrieve this crucial tribal leader. But once he arrives at the Circassian stronghold he encounters extraordinary impediments in addition to the anticipated warfare and political difficulties.
The other five books in this series are chiefly naval adventures but this one is different, other than the brief introductory interlude in the Black Sea near its conjunction with the Sea of Azov. Here, Phillip Hazard demonstrates that he is just as effective in leading soldiers on land as he is in commanding his war vessel. The author's painting of the characters, the landscape, and the numerous gripping events really pull the reader into the story which runs for almost 250 pages.
Vivian A. Stuart (born Violet Vivian Finlay) was born in 1914 and became a Lieutenant in the British Fourteenth Army during World War Two in Burma. She was decorated with both the Burma Star and the Pacific Star and she's actually written several series of military historical fiction (and romance novels) under multiple pseudonyms including Alex Stuart, Barbara Allen, Fiona Finlay, V.A. Stuart, William Stuart Long, and Robyn Stuart. Stuart was 72 years old when she died in 1986.
This is one of the finest adventure stories that I've read in recent times and can highly recommend it. If you enjoy tales of Cossacks and Circassians in the Caucasus Mountains then you might also like The Cossacks (Everyman's Library, #170) by Leo Tolstoy; Hadji Murad (also by Tolstoy); Mission to Circassia by Kathleen O'Dell, or; Lermontov's A Hero of Our Time (Penguin Classics).