Economics of the Roman army

Military burden

The size and growth of the Roman army of the Principate can be summarized from official unit strengths, excluding the Roman navy and foederati. The navy likely contained 30,000–40,000 personnel, with 15,000–20,000 in the Mediterranean fleets at Misenum and Ravenna; contrary to popular belief, warships were rowed by volunteer professional oarsmen, not convicts or slaves. Similar numbers served in the classis Britannica and fluvial flotillas on the Rhine and Danube. Substantial irregular tribal troops (foederati) were paid by the empire throughout the period; in Britain alone around 175 AD there were at least 5,500, and multiplying by ten for other frontier provinces suggests 50,000–60,000 irregulars at any time. At its peak under Septimius Severus, the standing military establishment may have exceeded half a million effectives. This burden was significant for a pre-industrial economy where at least 80% of inhabitants worked in agriculture. About 80% of the imperial budget in c. 150 was spent on the military; other sources suggest not less than 60%. Military spending constituted roughly 2.5% of the empire's GDP.