This work examines the political integration of the westernmost fringes of the Mediterranean in the Roman Empire, from the second Punic War (218 BCE) until the reign of Diocletian (305 CE). It aims to investigate the evolution of a particular geographical area over the centuries of its belonging to the Roman sphere, from the political inroads of Rome in the Far West until the second war against Carthage and the administrative reorganisation of these territories during the Tetrarchy period. A study of the westernmost Mediterranean region and its progressive incorporation into the Empire allows us to question and better illuminate the nature of the Roman power, as well as the administrative and military evolutions in these outlying areas. The political nature of the territories lying in the Mediterranean Far West underwent profound changes, from simple areas of influence to the constitution of provinces embedded in a vast network. To understand the structure of an empire and of imperial power, an examination of the margins, of the frontiers that it set and defended, is just as necessary as a study of its centre. According to conventional etymology the Latin expression nec plus ultra, literally “nothing more beyond”, adorned the pillars raised by Hercules on the shores of the Strait of Gibraltar to mark the boundaries of human control. Willing to support imperial policy and to celebrate the grandeur of the monarch, Charles V’s entourage introduced this new expression, drawing on the geographical myth of the creation of the Pillars of Hercules. The device of the sovereign, conqueror of the Atlantic world, thus became the famous Plus oultre or Plus ultra. This choice symbolised the transition from ancestral myth to the grandeur of the Spanish empire, direct heir of the Roman one. This appeal to an imperial heritage shows the ideological fertility of Imperium romanum as a reappropriation by the Spanish of the myths attached to the Far West, once the edges of the Earth and placed by the Renaissance at the centre of a new empire. Aside from being symbolically important, Iberia and Mauretania, crossing areas between two seas and two continents, maintained strong commercial and human relations following their colonisation by Phoenicians in the 8th century BCE. In 1959 the protohistorian Miquel Tarradell defined the Strait of Gibraltar in the Phoenician and Carthaginian eras not as a border dividing two seas and two continents, but as a bridge. Throughout Antiquity trade, human migrations and cultural influences have always existed between the southern Iberian Peninsula and the western Maghreb: some of their manifestations have been brought to light; others have been revisited, or even abandoned. But what about political relations between these Iberian and Mauretanian areas? How were they shaped on the eve of their integration into the Roman world? Was the Strait of Gibraltar a boundary, a chance or an obstacle to the making of the Empire? One of the main contributions of this work is thus to tackle the following questions: how geographically separate territories were jointly administered, and how a single authority was imposed and maintained on two continents, two seas and about ten different peoples. The last two centuries of the Republic and the first three of the Principate saw alternating periods marked by the simultaneous organisation of parallel structures on both shores of the Strait of Gibraltar and periods in which each area sought to forge direct ties with Rome. The other part of this enquiry examines the Greek and Roman conception of the Mediterranean borders such as Hispania and Africa Minor. The notion of nec plus ultra which gradually came to apply to the Strait of Gibraltar was not a ban for the sailors but a political statement of Rome asserting its claim to control the edges of the earth. Not only did the Romans describe the Strait, but they also conquered it. While current discussion of Mediterranean history, under the hallmark of “connectivity”, focuses essentially on social and economic history, anthropology or religious history, the subject of this book is imperial making and the part that the outlying regions might have played in the process. It centres on the “dark side” of connectivity, the control, appropriation and administration of the westernmost part of the Mediterranean. The study aims to show the stages of a conquest that was neither obvious nor necessary at the beginning. Strategic alliances with the Mauretanian kingdoms and the Far Western cities, political combinations and tactical needs gave rise to this movement of imperial expansion towards the Atlantic, which began during the Second Punic War and ended only under Claudius. Military and administrative arrangements were set as local circumstances dictated, pragmatically, with no overall plan or predetermined solutions. By contrast with a certain vision of Mauretanias developed during colonial period, the Far Western regions of the Mediterranean did not face military difficulties as important as other outlying regions, even if the military presence and expeditions remained important throughout the early centuries of the Principate. This integration of the Far West in the making of the Empire was completed with the creation of the diocese of Hispania during the Tetrarchy. This diocese, which regrouped Mauretania Tingitana and the Spanish provinces, was the first genuinely “Transfretan” administrative unit, formalising a de facto situation, that is the circulation of imperial power over both shores of the Mediterranean.