IT IS what a farmer would call 'very light land,'" the prime minister wittily told Parliament after the Anglo- French agreement of 1890. This was at the height of the colonial scramble for Africa. The prime minister was Lord Salisbury, the "very light land" the Sahara. Britain had generously agreed to give France a free hand in the largest, barest and poorest desert in the world. No wonder the Tory squires crowed with delight. Lord Salisbury continued sardonically: "We have given the Gallic cockerel an enormous amount of sand. Let him scratch it as he pleases."

The Conquest of the Sahara is the story of how that Gallic cockerel took up the challenge. Jules Cambon, governor general of Algeria, put it like this: "Well then, we shall scratch this sand, we shall put rails on it, plant telegraph poles, make artesian wells spring up from the soil." But for France as a whole the conquest of the Sahara proved as rewarding as the British expected. It added nothing except 4 million square kilometers of emptiness marked French on the map of Africa.

The rails of the projected trans-Sahara railway were never laid. Nor were there any minerals for France to exploit. Dreams of military glory proved merely dreams. This was hardly surprising as the defenders of the Sahara were only a few down-at-heel Tuareg armed with flintlocks and swords. In other parts of Africa the imperialists of the late-19th-century -- Louis Archinard, Cecil Rhodes, Leopold II -- seem a rapacious crew. In the Sahara it might look different: soldiers playing a game of sandcastles in the desert, innocent, boyish, absurd, the reductio ad absurdum of imperialism.

In this admirable book, Douglas Porch sets the record straight. Absurdity there was in plenty.But boyish innocence was rare enough among the French of the Sahara. Like most wars, the conquest of the Sahara was a butcher's business (though casualties were barely a thousand because of the ridiculously small numbers involved). Even the landscape proved unromantic: in fact it was appallingly dull and monotonous, apart from the moonscapes among the great massifs, the Hoggar and the Tassili. Where was the spirit of Beau Geste? No sign of it here among the French officers of the Chaamba camel corps raised to subdue the Tuareg. In fact both sides seem well-matched in bloody-mindedness -- if not in fire-power. Even decent officers were quickly brutalized by a war where the wounded were finished off after the battle. And many who fought in the French colonial army were jailbirds. Still the officers were professionals. They recognized that the real enemy was the terrain and the climate -- not the Tuareg -- and of course the killjoys back home. (If only he British imperialists had wanted the Sahara. How much easier that would have made things!) However, gradually the French officers learnt the lessons from their colleagues' blunders, of which there was no shortage.

THE FIRST great blunder was the expedition in 1880-81, led by Colonel Paul Flatters, to penetrate the Tuareg fastness in the Hoggar massif and cross the desert to Timbuctu. Despite 25 years in the Algerian garrison, Flatters seems to have made every mistake in the book. His guides were untried. He scorned diplomacy. He was led like a lamb to the slaughter. Most of the Europeans in his party were hacked down by the Tuareg. The survivors tried to struggle back to the north. Most of them died painfully one way or the other, the last European being shot and then eaten by his faithful companions. When the news reached France there was, predictably, a spasm of incredulity, followed by cries for revenge. Then Flatters was forgotten in the excitement of the Tunisian conquest, and rage at Britain's perfidy in Egypt.

It was nearly 20 years before officers of the French garrison in Algeria had another play at the central Sahara. By then French grand strategy included plans for an empire from Senegal and the Atlantic to Djibouti and the Red Sea, taking in the whole Sudan on the way. The British squashed this plan by calling Colonel Marchand's bluff at Fashoda, on the upper Nile. But the French bagged the western Sudan, as far as Lake Chad. Douglas Porch throws in this campaign for good measure -- and bizarre it was. The leaders of one of the three columns, Captain Voulet and Lieutenant Chanoine, got a bad dose of cafard (an occupational risk in the desert), tortured the natives on an unusually large scale, and then mutinied. They shot the French colonel who came to arrest them, before being polished off themselves. (This was not the way the British did things in Africa.) But the trail to Chad naturally led back to the Sahara.

The actual conquest took six years: 1899-1905. Despite Porch's efforts the story ends in a whimper. The commanders of the French columns, Theodore Pein and Henri Laperrine, were dull correspondents, if tough fighters. The Tuareg wisely avoided action whenever they could. The French won because they mastered belatedly the art of desert survival. Efficient camel corps were created. Victories were political. The Tuareg were won over by paying their leaders small salaries. It was the prosaic end to a pointless sideshow, away from the great drama of French colonialism in Africa and the east.

Pointless? For France perhaps. But not for Algeria. The irony is that the empty deserts around the Hoggar have now proved rich beyond the dreams of avarice. If it had not been for French intervention in the 1890s, their resources would now belong very probably, to Libya. Thanks to the imperialists, modern Algeria is not bankrupt, but enjoys the largest export trade in natural gas in the world.