THE ALLIED armies fighting in Italy in World War II included in their ranks so many varied and colorful units that they might well be compared to the Grande Arme'e of Napoleon that invaded Russia in 1812. Clark of the U.S. Fifth Army, for example, had under his command not only two famous Japanese-American battalions and one of the first black combat infantry divisions in the U.S. Army, the 92nd, but also a Brazilian Expeditionary Force and the Corps Expe'ditionnaire Francais, which consisted mainly of Moroccan mountain troops. British Eighth Army had an even richer diversity to show: Canadians, New Zealanders (including Maoris), South Africans, Ghurkas, Sikhs, Poles, Greeks and Israelis all fought, in their own formations, under the British flag. But perhaps the most extraordinary unit of all was the small, all-British raiding force known as P.P.A., initials which stood for Popski's Private Army.

It had been organized long before the Allied invasion of Italy, in North Africa, where the war was fought along the coastal road that connects Libya with Egypt; Italian, British and German armies advanced and retreated through towns that were taken and retaken -- Barce, Tobruk, Badia, Sollum and Sidi Barrani, to list some of the names that made the headlines of the English newspapers in those years. South of the road and its adjacent coastal strip lay mountains and the desert, a roadless, uncharted expanse of rock, sand and occasional marsh, inhospitable to large motorized formations and their logistical support columns. This was ideal territory for small raiding parties, a thousand-mile open flank from which the coast road and its wealth of ammunition and gasoline dumps, its airfields, P.O.W. camps, headquarters and communications centers, could be attacked with little or no warning.

Such operations however required specialized forces: men who knew the desert and the routes nomadic Arabs had followed for centuries and also equipment that would keep going over terrain that no one had ever imagined could be crossed by four-wheeled vehicles. It was the British Eighth Army that developed such units: the Long Range Desert Group, which ran regular convoys 1,500 desert miles from Cairo to Tripoli, the Special Air Service, which is still in existence and is now the British Army's principal force for special operations, and, last but not least, Popski's Private Army.

Its creator and commander was a British officer called Vladimir Peniakoff; he was nicknamed Popski by his fellow-officers, who found his name hard to handle on the field telephone. (Popski was a newspaper cartoon figure popular in England, a comic Bolshie with a huge black beard who carried a smoking bomb.) Peniakoff was born in Belgium; his Russian parents sent him to Cambridge, which he left to serve in the French army in World War I; when World War II broke out he was head of a sugar refinery in Egypt. He joined the British army as a second lieutenant; by the time the war ended he was a major in command of a force of over 100 battle-tested veterans who had fought their way from Cyrenaica to Tunisia and from Taranto to Venice on the enemy's flank or behind his lines, not as agents or partisans, but as uniformed, motorized, heavily-armed patrols which achieved with brilliant success the mission assigned them at their creation: to "spread alarm and despondency" in the enemy's rear. Popski lived long enough after the war to write the story of this legendary unit himself; Popski's Private Army was published by Jonathan Cape in London in 1950 and reissued in 1953 by the Reprint Society.

IT IS ONE of the most extraordinary books to come out of the Second World War. "My tale," Popski writes in his introduction, "is of war and hard work and enterprises, sometimes stirring but more often ludicrous; of sudden reversals of fortune . . . of lowly men of foreign nations whose devotion to our cause exceeded our own; of bloodshed and violence, but more of cunning and deceit and high spirits and the pleasant cudgelling of brains and then again more hard work; above all of friendship." The tale is well told; Popski writes with real talent, with an eye for the telling detail and a knack for the swift delineation of character; in this absorbing story his men and their exploits are brought to life in a narrative that commands belief by its understated account of heroic action and sympathy by its half-humorous appreciation of human failings, not least those of the writer himself.

Without realizing it, Popski had for many years been preparing for war in the desert. Fascinated by Arabs and the classic reports of such travellers as Burton and Doughty, he had learned their desert lore, as well as their language, from the Bedouin who roamed the Egyptian desert; later, in a Ford car fitted with balloon tires and a home-made sun compass, he made solo trips of hundreds of miles in the desert, teaching himself navigation and the arts of survival. When the war came he was ready with a project: "to build up a network of intelligence" covering the mountain flank of the enemy's base from Derna to Benghazi, "to take control of the friendly Arab tribes in that area and, as a minor objective, to destroy petrol dumps." The plan won official favor and the "Libyan Arab Force Commando" was formed under his command.

With one British sergeant, one Arab officer and 22 Arab "other ranks" he set out for "the enemy's back door," the mountain range that runs parallel to the coast from Derna to Benghazi. Though his spoken Arabic was far from perfect, his knowledge of Arab ways won him the support of the tribal leaders; from his base in their territory he set up an intelligence network to report all military movement on the road, destroyed an Italian dump of more than 100,000 gallons of gasoline and organized a mass escape of British prisoners from an Italian P.O.W. camp. The preliminary reconnaissance for this last operation was typical Popski: he simply walked through the town of Barce in broad daylight, calculating, correctly as it turned out, that his sun-bleached British khaki uniform would be taken for German by the Italians and vice versa.

Impressed by the success of his raids and the value of the intelligence he had gathered, Eighth Army authorized him to recruit British personnel for what became Popski's Private Army, its shoulder patch the letters PPA and its insignia an astrolabe, "a fitting symbol for a unit which would have to navigate its way by the stars." Its first assignment was in fact a long-range reconnaissance to find a route by which the German fortified line in Tunisia, the Mareth Line, could be outflanked by British armor. The route was found, but Popski's transport and its radio equipment was destroyed by the Luftwaffe. The way back to Eighth Army was too long for men on foot with little to sustain them; instead Popski marched his men 190 desert miles northwest to deliver the vital information to British First Army, advancing from its newly conquered base in Algeria.

With the fall of Tunis the desert war was over; there was "unfortunately," as one of Popski's men put it, "no war in the Gobi Desert." Popski began to plan for the war in Italy. He "wanted to take behind the lines patrols of five jeeps, each carrying a ton of supplies, mounting between them ten heavy machine guns -- the fire power of a battalion -- and with enough petrol for a range of 600 miles." And this is exactly what he did. From the heel of Italy to the Alps, his private army, infiltrated through the mountains or landed from the sea, was in almost continuous action, attacking enemy units from unexpected quarters, inflicting damage out of all proportion to their numbers, and, what is even more remarkable for a unit so steadily engaged in combat, suffering very few casualties.

Just before the end of the war, Popski achieved a cherished ambition, "a purposeless piece of swagger," as he says himself, but it was his "hour of triumph." PPA loaded five jeeps on a landing-craft, steered through the German mines to land at Venice, drove onto the Piazzetta and seven times round the Piazza San Marco.

He had not looked forward to peacetime; the war had been the high point of his life. "Up to the time I am writing about," he says in his introduction, "I had found little contentment . . . but during those five years every moment was consciously happy." With the German surrender Popski's occupation was gone. It is our good fortune that he found, in the few years left to him, another: to write this brilliant account of what was perhaps the most audacious and consistently successful of the many "special forces" created by the Allied armies in the Second World War.