Du Malone writes: Though studies of U-boat warfare typically focus on the Battle of the Atlantic, Lawrence Paterson — author of Steel and ice (The History Press, 2016) — reminds us that there were other arenas. Most of his study focuses on the north — the Baltic and Arctic. A couple of chapters, however, are devoted to the Black Sea.

Once Germany had decided to deploy U-boats to support the primarily land-based offensive against Russia, it faced the problem of how to get the vessels into the Black Sea. Eventually, it was decided to transport them from the north by dismantling them and then use a combination of road and river transport to reach the Romanian coast.

Once in the Black Sea, where the Russian fleet was dominant, the U-boats encountered difficulties. A major example was the inappropriateness of the pack tactics, developed in the Atlantic, for the largely coastal war in the Black Sea.

Nevertheless, The U-boats achieved a certain amount of success, both in terms of destroying and damaging shipping and, restricting the movement of the Russian fleet, which in turn restricted its ability to support Russian forces on land.

Eventually, though, events ashore — with the Russians advancing through Romania and Bulgaria — led to the flotilla receiving its final command: ‘Scuttle!’ — an order that was followed through close to the Turkish coast.

Submarine activity in the Black Sea has often stirred the imagination of thriller writers and film-makers. This, though, was the first historical account I’d read of such activity.