On May 9, 1915, more than 11,000 British troops were killed or wounded in a single day during an ill-fated assault on an entrenched German position in Aubers, France. The reason for the disaster: a shortage of artillery shells to support the assault, particularly the high-explosive shells best suited for trench warfare. The resulting “Shell Crisis of 1915,” led to a major reorganization of the British government’s defense production system. It still wasn’t enough to prevent the fall of Prime Minister H.H. Asquith a year later.

The shell crisis of World War I reminds us just how hard it is to prepare for long wars. “In almost every war there comes a point when it's clear that the peacetime defense industry, even if it had been ramping up, is inadequate for the demands of war,” says Mark Cancian, a retired Marine Colonel who is now a senior adviser at the Center for Strategic and International Studies (CSIS).

More than a century later, the world, and warfare, have changed. But even in a world of fighter jets, nuclear weapons, satellite communications, drones, and AI, ammunition still matters. The decline of the British Empire was followed by the rise of the United States as Earth’s military superpower, the Cold War's onset and end, and numerous conflicts throughout the Middle East. Now, America faces an arms shortage of its own. Even given the historical precedent, the current U.S. predicament is stunning.

The United States spends more on defense than the next seven countries combined, including rivals China and Russia. And yet, the U.S. military finds itself concerned about running short of key munitions in a war with a country, Iran, that could generously be described as a “middle” power.

In August, Reuters reported that the United States had used up “virtually all” of its long-range precision weapons during five months of fighting with Iran. The U.S. also reportedly used more than two-thirds of its supplies of Patriot interceptors in the Middle East, and the global Patriot shortage has in turn left Ukrainian cities dangerously vulnerable to Russian strikes.

President Donald Trump has angrily insisted that the U.S. has “virtually unlimited amounts of Mid to High Grade Ammunition.”However, the Pentagon’s own accounting tells a different story. An inspector general’s report in September admitted that the war had revealed “strategic inventory shortfalls.”Warnings from his senior military commanders about the state of U.S. stockpiles were reportedly one major reason why Trump has refrained from a return to full-scale combat against Iran.

If that’s what a few months of combat with Iran does to U.S. stockpiles, it’s hard to be optimistic about a potential war with China. A CSIS report warns that the U.S. would likely run out of some types of long-range missiles after just a week of combat in a war over Taiwan.

All of this begs the question: How is it possible that the most expensive military in the history of the world doesn’t have enough ammunition to fight a war?

The origin of the current munitions miseries is often traced back to a particular night in 1993, when Deputy Secretary of Defense William Perry invited defense industry executives to a secret dinner at the Pentagon, which came to be known as the “Last Supper.”Perry warned the executives that Cold War-era defense budgets were no longer sustainable and predicted that many of them would likely go out of business.

Perry’s prediction proved correct. Between 1988 and 1997, defense budgets fell by around 30% and about a dozen major U.S. defense contractors consolidated into four. Much of those budget cuts were taken out of funding for new weapons as the military drew down on Cold War-era stocks.

But the issue wasn’t just money. In the decade following the 9/11 attacks, U.S. defense spending more than doubled, though even amid growing tensions with China and Russia, it’s still less than half of 1980s budgets. But even with budgets creeping up toward a trillion dollars, dividing up the pie is harder than you’d think. Operations costs, maintenance, and spending on military personnel—including pay and retirement benefits—consume a significant portion of the U.S. defense budget. When it comes to what’s left, new high-budget platforms—aircraft, ships, planes—were often prioritized over munitions supplies.

“For many years, in internal defense budget deliberations, to find savings to meet the overall target, munitions were often the ‘bill payer,’” says William LaPlante, who served as Under Secretary of Defense for acquisition and sustainment in the Biden Administration.

The decisions were made easier by the fact that the wars being fought against insurgent groups in Iraq and Afghanistan didn’t require the same amount of missiles, shells, and air defense interceptors needed for a war against a nation-state military.

All that changed with Russia’s full-scale invasion of Ukraine in 2022, sparking a war featuring the kind of artillery use not seen in Europe since the days of the Battle of Aubers, as well as missile barrages on Ukrainian cities requiring a massive number of interceptors. All of a sudden, munitions were in high demand. After years of declines in America’s defense industrial base and manufacturing capabilities, increasing production of artillery shells as well as more advanced missiles and interceptors has been a frustratingly slow process.

What happened to the economic principle that supply rises to meet high demand? Defense companies have been reluctant to build out production capacity for new wars when the demand may disappear in a year or two. Indeed, it may even behoove some defense companies to stick to a high-demand, low-supply, high-margin business model. The impact for Americans, though, is that the cost to taxpayers stays high, even as the military struggles to keep up with the resource demands of modern combat.

Trump, for his part, has blamed Biden’s decision to supply Ukraine for the current shortages in the war with Iran. This is misleading: the vast majority of the weapons systems sent to supply Ukraine’s land war are of little use in the war in Iran. A notable exception is advanced air interceptors like Patriots, which are at a premium in both conflicts.

Both conflicts have made clear that the economics of using multimillion-dollar interceptors to knock down cheap cruise missiles and drones—akin to “throwing Ferraris at frisbees,” as Rep. Ted Lieu memorably put it—are brutally expensive for the defending side.

These days, the buzzwords for defense planners, as well as the crop of Silicon Valley-linked defense startups that have emerged in this post-Last Supper era, are systems that are “attritable,” meaning cheap and replaceable, and “modular,” meaning they can be easily adapted for future wars. Plans for the defense of Taiwan, for instance, involve building swarms of cheap and most likely AI-controlled drones to create a “hellscape” for Chinese invaders. But given how fast battlefield technology and tactics are advancing, even those plans may soon be obsolete.

“The whole system has to be shaken up because the move, counter-move cycle in modern warfare is eye-wateringly short, and we are not prepared,” says Mackenzie Eaglen, a defense analyst and senior fellow at the American Enterprise Institute.

In response, the Pentagon has announced a series of steps to speed up weapons acquisition by centralizing buying authority and cutting red tape. Though some of the plans have received pushback from Congress over the new powers being claimed by the department and concerns that it will incentivize companies to deliver inferior products quickly, it’s notable that virtually nobody is standing up to defend the old system—and changing this system will likely be a significant challenge in its own right.

Analysts often use the phrase “turning an aircraft carrier” to describe the challenge of reforming the bureaucratic behemoth that is the Pentagon and defense industry for a new age of warfare. The metaphor became literal earlier this year when the USS Ford was diverted from the Mediterranean to Venezuela and then back to the Middle East again as the administration jumped from one conflict to another. The ship would spend a record 326 days at sea—a mission that included food shortages, sewage leaks, and a nearly disastrous fire.

The logistical challenges of deploying the US Navy’s limited number of aircraft carriers, as well as concerns about munitions stockpiles, are reminders that even the world’s most powerful military is constrained by scarce resources, particularly given the growing number of wars it has been fighting lately.

The Pentagon has prioritized the production of 14 critical munitions in its spending priorities, but it’s still going to have to compete with big-ticket priorities, including Trump’s dreams for a $1.2 trillion “golden dome” missile defense system, a new focus on space-based weapons, and nuclear modernization. Even with a proposed $1.5 trillion defense budget, it’s going to be difficult to do all those things while fighting one, if not more, ongoing military conflict—as well as preparing for a potentially much larger one to come.

What if the problem is less how the U.S. prepares for wars than the wars themselves? “Restrainers” on both the left and right argue that the munitions crisis should prompt a rethinking of whether the U.S. should get involved in as many armed conflicts as it does and whether it can credibly commit to defending allies in a potential superpower conflict with China.

Money can buy you a lot of firepower, but it doesn’t exempt you from the need to make real choices about priorities.