The question of why Rome fell has generated more historical debate than almost any other topic in antiquity. Edward Gibbon blamed Christianity and 'barbarism.' Henri Pirenne shifted attention to the Arab conquests and the disruption of Mediterranean trade. More recently, Peter Heather has argued for the primacy of the Gothic migrations, while Bryan Ward-Perkins uses archaeological evidence of economic collapse to argue the fall was a genuine catastrophe rather than a 'transformation.'

The problem with mono-causal explanations is that Rome had faced every one of these threats before and survived. The republic defeated Hannibal. The early empire absorbed massive frontier pressures. Christianity had been the state religion for a century before the Western Empire collapsed. None of the usual suspects is sufficient on its own.

Guy Halsall's demographic and ecological approach adds another layer: the Hunnic pressure that drove Gothic migrations was itself a consequence of Eurasian climate instability. We're beginning to understand the role of the Antonine Plague and later the Plague of Cyprian in weakening both population and institutional capacity.

My own view is that the West fell to a convergence of structural vulnerabilities — fiscal overextension, the end of profitable conquest, elite tax avoidance, army professionalization away from citizen-soldiers — which then made it incapable of absorbing external shocks that earlier Rome would have handled.