It is hard to imagine today the degree of optimism everybody had about the war when Helmuth von Moltke, the German military commander, executed a version of the Schlieffen plan. Kaiser Wilhelm II told his soldiers: “You will be home before the leaves fall from the trees.” Young men left their jobs to go to the front. Older men, like the German sociologist Max Weber, lamented their inability to fight. In Britain, too, the general expectation was that the war would be “over by Christmas”. Trump predicted that the Iran war would last four to five weeks. We are now in week five.

Initially, in 1914, Germany’s war went according to plan. The Germans pushed through Belgium, encountering British forces near Mons, and then moved quickly over the border into France, pushing the French and British armies some 250km to the south. The Germans came within 40km of Paris. But the seeds of failure were already sown. They advanced, but they failed to surround the city. The quick encirclement – Schlieffen’s grand plan — did not happen.

As the Canadian historian Holger Herwig writes in The Marne 1914 about the Schlieffen plan: “It was a single roll of the dice. There was no fallback, no Plan B. Speed was critical; delay was death. Every available soldier, active or reserve, was deployed from the first day of mobilization. The sounds and sights of two million men trudging across Belgium and northeastern France with their kit, guns, and horses in sweltering 30-degree heat, stifling humidity and suffocating dust was stunning, and frightening.”

And herein lies the problem with grand strategy: it unravels on contact with the unexpected. Reality has a nasty habit of sabotaging even the most carefully laid plans. As Herwig tells it, the German troops were simply not prepared for the summer heat.

The Franco-British counter-offensive started on September 6 with the Battle of the Marne, the single most consequential battle of the First World War. Essentially, the Allied Forces managed to split the two German armies, the one that came from the north towards Paris, and the other further east. On September 9, the Germans began a general retreat, and by September 12 they had fallen back to the Aisne in the north. This was the beginning of the trench warfare that would dominate the Western Front for another four years. It ended with Germany’s total exhaustion, militarily and financially.

Historians have examined in great detail why Germany, the military and economic superpower of its time, lost the war. But the most important reason was apparent in those first six weeks: there was no plan B. The Schlieffen plan did not account for the weather. Nor did it consider what might happen if the war dragged on: acute supply difficulties.

Donald Trump, too, has no plan B. In fact, it’s hard to decipher if he ever had a plan A — whether it was toppling Iran’s government, weakening its nuclear capabilities or crushing its regional influence. Certainly, the American decapitation strategy succeeded at a purely technical level; Khamenei was killed in the opening strikes. But Trump’s bombardment underestimated the enemy, who was swift to escalate matters. Iran closed the Strait of Hormuz and attacked the Gulf states allied to the US and Israel. And it hit the US where it is most vulnerable: its reliance on the global economy and the global financial markets. I concluded a while ago that the West is so dependent on imported raw materials that it is not in a position to fight any substantial wars – with countries like Iran, Russia or China. So it is proving.

This was also the case for Germany in 1914: it was very strong, but also extremely vulnerable to supply shocks. Germany produced around 70% of its food domestically, and was dependent on imports for fertilizers and intermediate goods. It’s a familiar story. Today, the West is running into shortages of helium and urea, essential for the production of semiconductors and fertilizer respectively.  

This is something military planners, then and today, rarely have in their blueprints. Von Schlieffen and von Moltke were members of the aristocracy. They were focused on the big picture. They had little interest in the grubby business of mines and heavy industry. Similarly, the US military did not consider that there might be supply shortages for radars or interceptors. If the likes of Vladimir Putin and Xi Jinping have one advantage over us, it is that they understand raw materials and supply chains at a much deeper level than we do.

At the weekend, things escalated further. The Houthis, who control large parts of Yemen, entered the conflict with an attack on Israel. If the Houthis were to close the Bab el-Mandeb Strait, where the Red Sea passes through to the Gulf of Aden, the world would be on the verge of its biggest oil and supply-chain crisis ever. Around 30% of global container shipping passes through the Suez Canal.

There are also important differences to the First World War. But these are not necessarily comforting. Russia’s invasion of Ukraine in 2022 also started with a miscalculation leading to a long war of attrition that is yet to conclude. But unlike Germany in 1918, Russia is nowhere near the point of exhaustion — precisely because it has sounder supply chains.

Europeans have recently been framing the debate about their security solely in terms of the Russia-Ukraine war. They fear a Russian attack more than anything else. A far more immediate vulnerability, however, is their energy dependence. There are plausible scenarios of acute energy shortages ahead. The greater threat from China is not that it sells us cheap cars, but that it might stop selling us rare earth metals.

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I always thought the Americans were strategically smarter than Europeans. That was indeed the case for the US-led postwar world order. But I didn’t factor in Donald Trump. He is clearly not a strategic global actor. And he is currently blowing up whatever goodwill he has in the Middle East on a war with no concrete goal or exit strategy. The best outcome right now would be one in which Iran agreed to reopen the Strait of Hormuz and not to pursue a nuclear program. But this is essentially a return to the status quo of 2015, when the Iran nuclear deal was agreed — probably Europe’s last foreign policy success story. As part of this deal, Iran accepted limits on its nuclear program, like a cut in its stockpiles of enriched uranium. It was Trump who left the accord.

If the goal of this war is to effect regime change, then, a ground invasion will be required. This may yet come to pass: the Pentagon is already preparing for such an eventuality. But to succeed, such an invasion would have to be of an order of magnitude bigger than Iraq. We are talking about First World War levels of troops. With reserves, Iran has around one million troops. If the US puts 10,000 “boots on the ground”, this is not going to result in regime change. And if, as is mooted, Trump were to invade Kharg Island in the Persian Gulf, Iran’s core oil export hub, the Iranian and the Houthis would hit back at oil and gas infrastructure elsewhere in the Middle East.

I am sure that US and Israeli strategists have more information available to them than even the best-informed commentators. But as the hard-fought experience of past wars shows, information does not prevent leaders from making mistakes.

Perhaps the single most important lesson of the First World War, then, is that you should not base your strategy on a single roll of a dice, as Herwig put it. Smart gamblers know that too. I always thought that, for all his many faults, Trump was a smart gambler. But in this war, which is far from over, the cards are stacked against him.

It’s not hard to see why. Military history teaches us that airpower alone is insufficient to achieve the kinds of aims the Trumpians have set for themselves. Unless the administration accomplishes what has historically proved impossible, it must either scale back its objectives or attempt a ground invasion of Iran. As a former US Army infantryman, who fought in what used to be called the War on Terror, I know one thing: the latter option shouldn’t even be on the table.

A ground war against the Islamic Republic, on Iran’s own terrain, will send American troops into hell on earth — or about as close as we mortals can get to it.

First, the sheer size of the ground force will be staggering. Any serious American invasion of Iran would likely rival or exceed the scale of Vietnam or the 1991 Gulf War, making it the largest US military undertaking since the Second World War. Iran’s land mass and population size alone would prove a daunting hurdle. The country is nearly four times the size of Iraq, and its population is more than three times larger.

Iran’s terrain is, in a word, punishing. Its topography is dominated by mountains, which by some figures constitute half of its territory. Unlike most nations, Iran’s urban centers are located inland, nestled amid mountains, and buffeted by uninhabitable deserts to the east and the south. This combination of Iran’s built and natural environments would mean that US forces would have to advance through numerous bottlenecks to even reach Iran’s military and population centers. And while Iran is in the middle of the pack in terms of population density, its capital, Tehran, teems with more than 15 million souls; a fight to capture the city against a determined enemy would amount to the largest urban battle in American military history.

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The idea of capturing and pacifying Tehran alone illustrates the near-impossibility of such an invasion. Using the troop density employed in the 2004 battle for Fallujah, Iraq, as a point of reference, suggests that it would require more than 600,000 soldiers — roughly the size of the US deployment during the Vietnam War. Hence, perhaps, Leavitt’s talk of reinstituting the draft. 

Looking to the invasion of Iraq in 2003, a similar invasion of Iran, given its population size, would require as many 1.6 million troops. Those estimates appear more daunting still when compared with the actual force structure of the US military, which boasts about 2.1 million members in active service, reserves, and the National Guard. Of these, only about 20% are combat troops. Simply put, committing 1.6 million personnel would mean using roughly three-quarters of the entire US military, including combat forces America simply doesn’t have.

Such a war couldn’t be undertaken without a massive mobilization, and a fundamental reordering of American global commitments, requiring the diversion of forces from Europe, Asia, and elsewhere. An invasion of Iran would therefore impel the United States to confront something it has long sought to avoid: hard choices about the limits of its global military footprint. Summing up the scale of the prospect, the columnist Max Boot, not exactly known for his pacifism, described an invasion of Iran as “the mother of all quagmires.”

Then comes the next obvious problem: fighting the war. While Iran’s military would likely lack air superiority and remains less technologically advanced than that of the United States, the Islamic Republic shouldn’t be taken lightly. Before the outbreak of the current war, the country’s regular armed forces were estimated at roughly 420,000 personnel. Iran’s more ideologically committed Islamic Revolutionary Guard Corps numbered around 190,000, backed by a militia force, known as the Basij, of more than 600,000.

How much of this force could ultimately resist an invasion is speculative: some may defect, and others removed from the battlefield before they can fight. But what is clear is that Iran would likely adhere to its long-standing doctrine of defense in depth. Drawing on lessons from the Iran-Iraq War, Iranian planners have adopted what they call a “mosaic strategy”: decentralized command structures combined with a willingness to trade territory to wear down an invading force through attrition.

Supporting this strategy would be Iran’s still-significant arsenal of rockets and drones. These have already surprised US military officials in recent engagements, and could prove even more dangerous on Iranian soil, where shorter flight times would give the Americans far less warning. Iranian militias in the region, too, are battle-hardened, versed in irregular warfare, and, in many cases, acquainted with American tactics.

US forces would likely find themselves fighting a determined asymmetric opponent armed with effective weapons and operating on difficult terrain. For a military that has not attempted a major invasion in more than two decades, and that remains only partially tested against modern drone warfare, such a fight would be far from easy. In short, Iranian commanders would aim to draw the United States into a long and bloody war of attrition, and they would have the terrain and urban density do it.

Some will argue that the past is not prologue — that the Iranian regime is unpopular and therefore unlikely to inspire serious resistance. It’s telling, however, that American bombing has yet to create fissures in the regime. Nor has the American air campaign inspired the Iranian people to revolt. On the contrary, all available evidence points to regime consolidation, and the closing of the gap between public sentiment and elite opinion.

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Then, too, critics underestimate the power of nationalism, particularly in the Iranian case. The “rally around the flag” effect — the well-documented tendency of citizens to unite behind their government in wartime — was most clearly demonstrated in Iran after Saddam Hussein invaded the country in 1980, a move that ultimately consolidated the very regime the Iraqi dictator had hoped to topple.

Even under the most ideal circumstances of state collapse — like those seen in the early days of George W. Bush’s Iraq War — a post-invasion insurgency in Iran would be all but certain. Between the most fervent supporters of the IRGC, and with the potential backing of Shiite militias across the region, it would not take many committed fighters to prolong the conflict. As Washington learned in both Iraq and Afghanistan, even a relatively small but determined insurgent force can sustain a war for years. That force could draw from an estimated “fit-to-service population” in Iran of 41 million, plus the larger Shiite sphere, including warriors from Iraq, Bahrain, Lebanon, and Pakistan, among others. Much like Iraq and Afghanistan, the defeat of the Iranian army would be a fleeting victory assuredly followed by a bloody and open-ended slog with an insurgency. 

Finally, an American escalation into a conventional ground invasion would deepen the already daunting regional crisis. Deploying ground troops in Iran would provide an ample opportunity for America’s great-power rivals, Russia and China, to drain Uncle Sam of blood and treasure. So far, the pair have yet to expand their assistance to Tehran beyond diplomatic and intelligence support. But the presence of American forces on Iranian soil would provide Moscow and Beijing with a tempting pressure point against a superpower that is already overextended, indebted, and domestically fractured. Iran, then, could become a bloody site of great-power competition. 

Given the totality of Iran’s geography, demographics, military capability, and geopolitical connections, the prospect of an American invasion of the country should be unthinkable. Yet Trump’s ill-considered decision to launch the war, coupled with his vague-but-ambitious goals, has made this impossible scenario a military possibility. Given the horrific costs such an invasion would entail, however, Trump should choose a different path: declare “victory” and de-escalate. Washington must stop throwing good American money after bad — and preserve the lives of the brave men and women who daily underwrite our national security with their blood.