This article studies the psychological, educational, organisational and structural factors that wedded the French conquest of Algeria (from 1830)to the conquest by the Romans of the same territory and the same tribes. The Roman Empire had expanded beyond Europe to (present day) Turkey, Lebanon, and Syria, through Libya and Tunisia to Algeria and Morocco, thereby turning the Mediterranean into a Roman lake, and leaving plentiful monuments in the conquered, romanised lands. The later Byzantine empire extended over the same territories. Although 'modem' war might seem much removed from Roman conquest, the conditions the French met in Algeria were similar to those faced by the Romans, with the addition of 'gun-powdered weapons. Except for their invasion of Egypt, the French had had little experience of fighting mounted tribes in desert conditions, with over-stretched lines of communication far from France. They needed accounts of the Roman conquest as practical intelligence in spite of the time-gap since the 'reports' were relayed by the ancient authors. They saw their conquest as the beneficent gift of civilisation, and they found themselves, quite consciously thanks to their classical education, follo~ing not just Roman tactics, but re-using Roman and the later Byzantine forts, bridges, roads and aqueducts, and frequently their lines of communication as well. Just as it is perfectly possible today to drive around Tunisia or Algeria using a map of the Roman roads, so the French quickly discovered that the remains of ancient wars of occupation were at first vital to their survival. What is more, unavoidable facts of geography and hydrography ensured that the French followed the Romans' settlements, communications and water supplies. They were encouraged by the plentiful Roman remains to believe that semi-deserts could once more be made fertile.