Allies that depend on U.S. military equipment need a Plan B.

Although the United States has been the world’s preeminent military power for decades, it apparently cannot fight a regional power such as Iran for more than a few months before experiencing an armaments crisis. Since hostilities with Tehran began on February 28, the U.S. has reportedly used up two-thirds of its Patriot missile interceptors, more than a third of its Terminal High Altitude Area Defense (THAAD) interceptors, and a third or more of its prewar stock of Tomahawk cruise missiles. The rate of consumption is all the more striking because a shaky cease-fire had halted most fighting for months. Based on recent production patterns, replenishing the supply of weaponry will take several years.

Making America’s challenge still more daunting is a major shift in war-fighting technology toward cheap, unmanned aerial vehicles. The United States and its allies in the Persian Gulf have been forced to use expensive, advanced air-defense systems—such as Patriot PAC-3 missiles that each cost millions of dollars and have a long production cycle—to shoot down Iranian drones that can be swiftly assembled for about $30,000.

The munitions shortfall doesn’t just affect the current war. It also sends a worrisome message to allies around the world: The United States has been caught off guard by the latest evolution of warfare and lacks the armaments-production capacity to adapt quickly. Suddenly, relying on American military technology looks far less reliable as a security strategy than it did before President Trump went to war with Iran. And in the long run, governments that counted on the United States to equip their military are going to need a Plan B.

In the short term, concerns about diminished weapons stocks are already limiting Washington’s strategic options. Because of his advisers’ concern about the adequacy of current supplies, Trump has reportedly been hesitant to escalate the war. If the United States decided to send ground forces into Iran on the scale necessary to keep the Strait of Hormuz open, defending those forces could exhaust the remaining stocks of advanced air-defense systems. After a short period, American troops would be far more exposed, and casualties far more likely.

Trump and his appointees during his current and previous terms bear only part of the responsibility for the current situation. The fundamental problem has been a bipartisan neglect of America’s military-industrial productive base over the past three decades. The United States allowed many of its strategic industries, such as shipbuilding, to wither. Its post-9/11 wars in Iraq and Afghanistan involved brief invasions followed by years of relatively low-intensity counterinsurgency warfare that, whatever its difficulties, did not require mass stockpiles of munitions.

Informed by these experiences, military planners appear to have been operating under the assumption that future wars would be short and containable. If China attacks Taiwan and the United States wants to come to the island democracy’s aid, the Pentagon will not be prepared for a protracted war.

Unfortunately, the Trump administration has failed to learn from the war in Ukraine. Iran has learned to mass-produce cheap drones—ones that the Russians first imported and now produce themselves. In other words, their effectiveness had been tested, and their existence was widely known. But that seemed to make no difference to U.S. planners. Perhaps in the arrogant belief that a war with Iran would quickly end in a decisive victory, the Pentagon did not have cost-effective systems ready to intercept these weapons. Neither did the Gulf countries that have counted on American weaponry for their self-defense.

In contrast, Ukraine has proved remarkably adaptive to emerging conditions. Creating a powerful strategic industry, as Ukraine has done in the case of air, land, and sea drones, not only helps with the country’s security. It also provides a boost to Ukraine’s economic and strategic position, as governments in the Gulf and elsewhere seek to tap its expertise.

The Gulf partners that build their militaries around equipment from the United States do so under the assumption that, in a moment of need, they can get what they require from the superpower. Perhaps the U.S. would even carry the burden of any war in the region. The perils of such assumptions are becoming apparent.

Relying on any other country for weapons is dangerous. Doing so may create a flashy frontline force with some advanced capabilities. But once the purchased equipment is destroyed or worn out, the military that bought it is left with troops trained on equipment that it cannot make and with a military doctrine built around equipment it might never have again.

Sometimes allies drag their feet or change their mind. Ukrainians have used American military equipment to great effect, but the unreliability of those supplies—because of President Biden’s fear of antagonizing Russia and Trump’s usual favoritism toward Vladimir Putin—has hampered their war effort.

Since Trump’s return to office, traditional European allies have come to recognize from his criticism of NATO that they will need to rely more on their own resources for their security. The current munitions crunch could lead its Gulf partners to the same conclusion.

If Saudi Arabia and other customers of the American defense industry start sourcing their military equipment elsewhere, or making it themselves, the United States will lose out. Beyond improving America’s military position and diplomatic leverage, arms exports have been a lucrative industry.

If the United States appears less powerful and less reliable than it once was and is running through munitions faster than it can produce them, many more countries will follow Ukraine’s example. The Pentagon is belatedly ramping up its efforts to develop less expensive self-guided weapons. But those initiatives so far seem inadequate to address the problem that the war with Iran has revealed: a gap between America’s military needs and what it is capable of manufacturing quickly and cheaply.