Your book is a history of Roman legions. What is their greatest legacy?
Organisation was certainly the key to Roman military success - and not only on the battlefield. From recruitment, through equipping and training mass enlistments of recruits, to swift transfer of units to trouble spots, to logistics in the field, the Romans' organisation was regimented and thorough.

The other legacy was strict discipline. In the legions, military law prevailed and discipline was brutal, with a long list of service regulations enforced by centurions' vine sticks and capital punishment. Discipline on the battlefield kept units obedient, intact and fighting, even when the odds and conditions were against them.

Is the influence of the legions felt in the way modern national armies are organised?
The organisational structure of today's armies stems directly from that of the Roman legion. Similarly, a self-contained battle group that does not have to rely on outside support, as was the case with the legionary armies of the 1st and 2nd centuries, is an ideal military force today.

Can we draw any parallels between the legions of the Roman empire, and the US army and the American imperium today, such as it is?
The US military has come to rely on superior numbers, superior technology and superior firepower. Occasionally the legions did possess all three, but more often than not they didn't - yet still they won enough battles to keep the empire intact for several centuries.

Modern wars such as those in Vietnam and Afghanistan today have shown that, in a conflict where you can't always be sure who your enemy is, numbers, technology and firepower can be meaningless. If several Roman legions were dropped into Afghanistan today, they would be brutal, bloody and ruthless. But they were trained for set-piece battles, as are armies today, and in the end the nature of insurgent warfare would defeat the legions.

In the introduction to Legions of Rome, you seem to attribute the "disintegration" of the Roman empire to the withering of the imperial legions.
Numerous fierce neighbours are bashing down your front and back doors and sneaking in the windows. Your equally fierce family is capable of throwing them out again, but the men of the house are frequently distracted by fighting among themselves. The injuries caused by infighting reduce the householders' ability to resist the threats from without. What is the house's future?

Agreed, the empire's decline in the west was due partly to political and economic factors, but the decline in military strength was all-important. Who knows how much longer the empire would have lasted had there not been so many civil wars that drained treasuries, removed able leaders from the stage and cut swaths through the Roman military ranks?

To what extent, if any, is your account of the Roman decline influenced by Gibbon?
I do quote Gibbon, but only half a dozen times in 200,000 words, and then usually to repeat a sage observation. Decline and Fall of the Roman Empire is not a particularly useful resource for information about specific legions. Nonetheless, Gibbon's years of labour have always inspired me.

Which other historians of the Roman empire do you admire?
Tacitus, first and foremost. It is true that he was both a snob and a misogynist, but as a recorder of Roman people and events of the 1st century, and of the movements and battles of individual legions, he is unparalleled. There are also many fine British historians today whom I admire, people doing excellent work on the minutiae of Roman military life.

Do you have a favourite legion?
The XIV Gemina Martia Victrix Legion, which rose from shame during Julius Caesar's day, after walking into a trap in Gaul and being wiped out, to fame, gained during the 60-61AD Boudiccan revolt in Britain by forming the mainstay of the vastly outnumbered Roman army that defeated the rebels. The mere mention of the legion's name after that victory was enough to make opponents quake in their sandals.

Interview by Jonathan Derbyshire
Stephen Dando-Collins's "Legions of Rome: the Definitive History of Every Roman Legion" is published by Quercus (£35)