Armies are one of history’s great paradoxes. They are institutions of immense power, capable of creation and destruction on a continental scale, yet we often fail to see them as what they are: distinct historical phenomena, born of specific crises and ideas, that profoundly shape the societies that create them. An army is not just a tool of the state; it is often a mirror, a crucible, and a motor of social and political change.

This is the central theme of a fascinating conversation on the Explaining History podcast with Barney White-Spunner, a former senior British Army commander and author of the new book, Nations in Arms: Five Armies That Made Europe. The book takes a long view, from the Roman legions of Constantine the Great to the American GIs of 1941, to explore how certain armies, at pivotal moments, did more than just win battles. They redefined their nations.

The Revolutionary Template: Armies Born of Crisis

White-Spunner’s chosen armies—those of Constantine, Mehmed the Conqueror, Oliver Cromwell, the post-Jena Prussians, and the WWII-era United States—share a common thread. They were not products of gradual evolution. Instead, they were radical reinventions, forged in the heat of existential crisis and built from first principles.

Take the Prussian military revolution after its humiliating defeat by Napoleon at the Battle of Jena in 1806. As White-Spunner explains, reformers like Gerhard von Scharnhorst and Carl von Clausewitz did not just tweak the existing model. They went back to basics, creating a new kind of army built around the famous concept of Auftragstaktik (mission-type tactics). This doctrine of delegating decision-making downwards, empowering junior officers to exercise initiative based on the facts on the ground, became a hallmark of German military effectiveness for over a century. The Prussian General Staff became an intellectual powerhouse, a “brain” for the military that was revolutionary in its time. The historian Christopher Clark, in his masterful Iron Kingdom: The Rise and Downfall of Prussia, details how this military reform was inextricably linked to wider social and political reforms aimed at creating a modern, mobilized nation-state.

This pattern of crisis leading to innovation is echoed in Cromwell’s New Model Army. As the podcast discusses, radical Parliamentarians in the 1640s, frustrated by indecisive aristocratic leadership, demanded a “winning machine.” The result was a revolutionary force, meritocratic and ideologically motivated, that not only defeated the king but became a hotbed of radical political thought. The famous Putney Debates of 1647, where common soldiers argued with their generals for universal male suffrage, were an extraordinary, world-changing moment—a direct product of the army’s unique social composition. The historian Christopher Hill, in The World Turned Upside Down, brilliantly captured the explosion of radical ideas that the New Model Army both embodied and unleashed.

The American Colossus: When an Army Becomes a Superpower

Perhaps the most staggering example of military transformation is that of the United States. In 1941, the US Army was, as White-Spunner notes, a third-rate power, ranked “somewhere after Portugal.” Within three years, it had become a global colossus, fielding over 80 divisions across two theatres of war. This was not just a triumph of industrial production—what StalinStalin Joseph Vissarionovich Stalin (18 December 1878 – 5 March 1953) was a Soviet politician, dictator and revolutionary who led the Soviet Union from 1924 until his death in 1953. Read More called America’s “country of machines”—but also a triumph of intellectual freshness.

As the podcast highlights, at the Casablanca Conference in 1943, the American chiefs of staff, led by George C. Marshall, began to assert their strategic vision over their more experienced but tradition-bound British allies. They brought what the host calls a “beginner’s mind” to the immense problem of winning the war. This “unprecedented and unparalleled” mobilization, as the historian Rick Atkinson chronicles in his Liberation Trilogy, did more than win the war; it fundamentally remade America’s relationship with the world and cemented its status as a superpower for the next half-century.

The Double-Edged Sword: When Armies Rule

However, Nations in Arms also contains a crucial warning. An army that is successful in defending its society can become a threat to it. The politicization of Cromwell’s New Model Army, for all its democratic potential, ultimately led to a military dictatorship under the “Rule of the Major-Generals,” one of the most disastrous periods in British history. As White-Spunner forcefully argues, “armies do not run societies well.” The ultimate lesson, learned by Cromwell’s successors, was that an army must be subordinate to civilian political authority, whatever its form.

This is a lesson that Hitler, fatefully, never learned. As the podcast points out, he systematically dismantled the very tradition of Auftragstaktik that had made the German military so formidable. Believing in his own myth as the “inspired leader,” he micromanaged his generals from afar, leading to catastrophic defeats from Dunkirk to Stalingrad. This illustrates the fatal flaw in the fascist model of leadership: it replaces professional competence and rational analysis with the supposed infallible will of a single man. The historian Ian Kershaw, in his biography of Hitler, details this process of self-delusion and its disastrous military consequences.

By examining these pivotal moments, White-Spunner reveals the complex and often contradictory relationship between armies, states, and societies. An army can be a vehicle for revolution and social progress, but it can also become an instrument of tyranny. As we face a world of new and complex threats, from pandemics to climate change, understanding this history is more important than ever. It forces us to ask fundamental questions about what armies are for, and how a nation can be defended without losing its soul.

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Further Reading

• Barney White-Spunner, Nations in Arms: Five Armies That Made Europe: The book at the heart of the discussion, offering a deep dive into these five pivotal military transformations.
• Christopher Clark, Iron Kingdom: The Rise and Downfall of Prussia, 1600-1947: The definitive history of Prussia, providing essential context for the post-Jena military reforms.
• Christopher Hill, The World Turned Upside Down: Radical Ideas During the English Revolution: A classic work of social history that explores the incredible explosion of political and religious radicalism in Cromwell’s England.
• Rick Atkinson, The Liberation Trilogy (An Army at Dawn, The Day of Battle, The Guns at Last Light)*: A Pulitzer Prize-winning narrative history of the American army’s campaign in the European theatre of World War II.
• Andrew Roberts, Masters and Commanders: How Four Titans Won the War in the West, 1941-1945: A brilliant analysis of the strategic debates and personality clashes between the Allied leaders, including Roosevelt, Churchill, Marshall, and Brooke.
• Ian Kershaw, Hitler, 1936-1945: Nemesis: The second volume of Kershaw’s masterful biography, detailing Hitler’s descent into military megalomania and the ultimate downfall of the Third Reich.