The question I am asked most frequently about my new book, Nations in Arms: Five Armies That Made Europe, is why I chose those five armies – being the armies of Constantine the Great and Mehmet the Conqueror, Cromwell’s New Model Army, the Prussian Army after Jena and the American army from 1941 to 1945. Part of the feedback from the book will, I hope, be readers saying that I have got it all wrong and I ought to have selected a different cast. I will probably disagree with them but then the debate should be fun.

I assessed the five armies I chose against a set of criteria – their ability to field an effective combat force, their ability to innovate, the way their soldiers were regarded and cared for by the societies that spawned them and their nation’s industrial base. Lastly, but arguably most importantly, is the contribution they made to their nation or empire; an army’s role is to defend its citizens, their interests and property and to be true to the societies from which they were sprung. They should be servants of the nation, not of some faction or individual who may usurp them for their own ambition, ultimately leaving their country weaker.

Why did I take Constantine’s army rather than earlier Roman armies, such as that, for example, of the mighty Julius Caesar? Caesar’s army was undeniably successful, in its early days furthered the interest of Rome, was certainly innovative and his soldiers were all Roman citizens. Yet it became an army that was used to serve the interests of one man and, from Caesar’s assassination in 44 BC, those of his family. It was an army that was used as much to fight civil wars as it was to serve Rome – I often find myself wondering how leaders could motivate their soldiers to fight each other, presumably men they had previously fought alongside? It also slowly began to stagnate so that its tactics and procedures became a weakness, leading to its disastrous decline in the mid-third century. Constantine’s army can also be fairly said to have been created to serve the interests of one man and to fight civil wars to establish his hegemony. Yet behind Constantine’s undoubted personal ambition there lay a nobler cause which would lead to the overall betterment of the empire. Constantine was certainly not a Christian crusader, as his eulogising Christian biographers have portrayed him, but he saw the utility of Christianity in providing a social structure which would give some stability, thus allowing the development of agriculture and industry. It was the reformed army he created that allowed him to do this.

I am sure there are other armies even before Constantine who might fulfil my criteria, possibly in Asia, and Alexander the Great’s army deserves serious consideration, except that it would appear to have lacked a motive other than conquest for the sake of conquest. Also, we don’t know very much about it, which makes it difficult to argue for its inclusion. However, the question I am asked most persistently is where is Napoleon’s army? My answer is that although it was undoubtedly one of the most innovative and successful armies ever fielded, how did it serve France as opposed to the emperor who led it? And it is interesting that I am always asked not why I had excluded the early nineteenth-century French army, but why had I not included Napoleon’s, the personalisation itself justifying my decision. France in the eighteenth century was arguably the most powerful country in the world, certainly the most powerful in Europe, with one of its largest populations, which grew by 50 per cent throughout the century. After Waterloo, France stagnated. By 1850 its population was only marginally higher than in 1800, and by 1900 it had grown just 25 per cent compared with Germany and Great Britain, which both grew by 300 per cent, although admittedly the German figure is slightly misleading because Germany had not existed prior to 1870. France mounted a disastrous expedition to Mexico, a marginally less disastrous one in the Crimea and was then comprehensively defeated by Prussia. Its domestic politics remained chaotic. Arguably France took over a century to recover from Napoleon, some would say even longer. The French may have benefitted internally from Napoleon’s administrative reforms but militarily and internationally, after twenty-three years of his wars, it had gained nothing.

I included the British New Model Army for two reasons. Firstly, it is where the contemporary British army came from. Much of how we are organised and how we operate today is purely Cromwellian. But, secondly, it is an excellent example of how an army can try to usurp civilian power in a manner which only leads to – near – disaster. That it pulled back from the brink and survived, and how that happened, is worth a book in itself. Similarly, the Prussian army created by Scharnhorst and his team after Prussia’s terrible defeat at Jena in 1806 is, to my mind, probably the best example of how an army should be structured; that it would be corrupted first by the German Empire and then the Nazis is one of the tragedies of modern European history. An army created from the best of motives, and a commendably efficient and popular army, became the army of fascism.

Maybe the most interesting of my five is the quite remarkable story of how the army of the United States, ranked eighteenth in the world in 1940, so somewhere below Portugal and Bulgaria, was transformed in three years so that by 1944 it was fielding over sixty divisions in Europe and over twenty in the Pacific. Taken together with the Prussian army, it offers some excellent lessons for modern armies, and today’s US army is in effect the same army – the most powerful and technically proficient ever fielded. Other armies I have been asked about are the Swedish Army built in the Thirty Years War by Gustavus Adolphus, undoubtedly extraordinarily competent but which, after its subsequent disastrous defeats at Ferhbellin and Poltava, left Sweden on the margins of European power for centuries. And why have I excluded the Russian army? To which I answer, ‘Which one?’ The Soviet Army was not the Tsarist army, although both relied on mass and drew on Russia’s huge reserves of manpower in a way that was effective only if you discount the subsequent slaughter. Stalin had little choice in 1941 but the Russian army’s operations in Ukraine, similarly expensive in Russian lives, can hardly be said to be advancing the interest of Russia’s citizens.

This was a fascinating book to research. I wrote it because I wanted to show that the western world has confronted many crises over the centuries. Resourceful rulers have met these crises by thinking through exactly what an army should be, often under tremendous pressure. None of them concluded they should do more of the same, but rather that new threats required new solutions. They offer some sanguine lessons for governments today.

Get your copy of Nations in Arms: Five Armies That Made Europe here.