This is one of those "Bantam War Books," which I used to see everywhere back in the 60s and 70s. Now, of course, they're harder to find! This book was originally published in Britain in 1961; this Bantam edition was published in 1981. It's about British Commandos who had been formed into "Commando Force 133" to hold an island in the Adriatic Sea called Vis. Needless to say, I had never heard of "Vis," but it is one of the islands off the coast of Dalmatia, the farthest one off the coast. During WWII, it was a part of Yugoslavia; now, I think it's part of Croatia.
The British on Vis were working with Yugoslav Partisans under the command of Tito. The relations between the British and the Yugoslavs makes for one of more interesting aspects of the story. Both groups were composed of tough soldiers but the Yugoslavs did not care much about discipline and the Brits, as we could expect, were highly disciplined...Another difference was that the Partisans included women, some very tough women.. Together, the commandos and partisans made raids on the German-held islands off the Dalmatian coast as well as prepared for a German attack on Vis.
The story focusses on Doctor Jim Rickett, who tried to cope with the casualties with a poorly equipped hospital. He was able to improvise and actually set up his hospital using parts from shotdown bombers.
There is quite a cast of colorful characters, such as "Mad Jack" Churchill, a Commando officer. For the Yugoslavs, there's Madame Zena, a partisan leader and "Chicago Mary", a dedicated Communist ( who had lived in Chicago). There are several Americans who appear in the story, as there was a team of Army Rangers attached to the Commando Force.
I checked out some info about the author, Bill Strutton (1918-2003). An Australian, he had served in the Australian Army on the island of Crete, where he was captured by the Germans. As well as a novelist, he was a screenwriter for such British television shows as "Doctor Who" and "The Avengers."