The Romans invaded Britain in A.D. 43. In the early part of the second century A.D, much of Londinium-the Roman outpost city that eventually became modern London-was destroyed by fire There is no contemporary reference to this great fire, or possibly succession of local fires. However, archaeological evidence leaves no doubt that such a disaster did occur between A.D. 120 and 130. The Hadrianic fire (named for the Roman emperor of that time) marks the onset of a profound change, and in some respects a decline, including a substantial fall in the population of Londinium after about AD. 150.

At its greatest extent in the early second century, the population of Londinium may have been in the region of 30,000-greater, perhaps, than it would be again for over 1,000 years. Recent estimates of the population of Roman Britain as a whole (i.e., of England and Wales), range between almost three million and up to six million. All such estimates must necessarily be uncertain and partly speculative, but they do at least give some idea of the scale of the matter. The population of Londinium, it may be noted, amounted to at most 1 percent (or 0.5 percent if the figure of six million is used) of that of Britain as a whole, far below the ratio of later times-in 1550, for instance, 2.5 percent of the population of all England, and in 1700, 11 percent. In terms of the likely numbers of its inhabitants, it seems therefore that even at its peak, Londinium's position in relation to the provinces was in Roman times less important than it was to become later.

Despite the extent of the damage that it caused, the Hadrianic fire of A.D. 120-130 cannot have been the main cause of the recession of Londinium. Nor can the frequent military operations in North Wales and southern Scotland, which took place during the Antonine period (A.D. 138-192) account for it, although they may have drained limited resources away from the southern part of the province and may even, by often requiring the governor's presence on the northern frontiers, explain the reduction in size of his palace in Londinium. In Britain the favorable commercial opportunities of the first century-when the potential of the new markets that existed there could most easily be exploited-were over by the mid-second century. As the Romanization of the province advanced, this colonial nature of its trade declined. The first frenetic phase of the Romano-British economy had passed, and things were settling down. Most trade was becoming local, centered on the tribal capital of each region. Home-produced goods began to take the place of imports, and Londinium's function as the principal center of trade both within the province and with the rest of the empire began to diminish.

However, this changing pattern of trade does not by itself provide a complete explanation for the great fall in population of Londinium around AD. 150, perhaps by some two-thirds. Still less does it explain the virtual termination of production in about AD. 160 at the potteries at Brockley Hill and Highgate, which until then had commanded a large market in Londinium, or the abandonment of the area beside the Walbrook River within the city. Demographic decline is nevertheless well attested by the numerous deserted buildings; by the deposits of dark earth, which often imply the practice of gardening or farming; and by the much smaller quantities found by modern archaeologists of building remains and general domestic bits and pieces relating to the years after about AD. 150 The third quarter of the second century, when the depopulation of Londinium seems to have been at its height, coincides with the Plague of Galen, an epidemic brought back by soldiers returning from service in the East that swept across Europe in AD. 166-167.Londinium, with its frequent contacts with Gaul (modern France) and Germany, would have been the natural point of first entry for any epidemic, and the fall in population that began there seems to have been followed in the early third century by a similar decline in other parts of southeast Britain