When the United States began bombing Iran, it expected to quickly overwhelm the country’s government. In the war’s first 24 hours, American and Israeli forces hit roughly 1,000 targets. Over the following 37 days, they blew up 12,000 more, laying waste to much of Iran’s military infrastructure. The strikes killed Iranian Supreme Leader Ali Khamenei, Iran’s minister of defense, and the head of the Islamic Revolutionary Guard Corps.

But more than six months later, the Iranian regime not only remains intact; it is perhaps stronger than before. It immediately replaced lost officials and struck back at Washington and its regional partners. And it shuttered the Strait of Hormuz, driving up energy prices around the world and winning newfound political leverage—all to the great frustration of U.S. officials.

There are several reasons why Iran performed surprisingly well. For one, the country had long planned for American and Israeli attacks, so it was prepared to reconstitute its government when top leaders were killed. It was also able to import enough goods to keep its economy from completely collapsing despite a U.S. naval blockade. But one of the most significant factors is that the Islamic Republic had built up a large arsenal of cheap yet accurate drones and missiles. These munitions made Iran’s counterattacks effective and allowed it to close the strait—and thus deter American escalation.

In doing so, Iran has demonstrated something broader. The age of what I called precise mass—or high-volume, low-cost, precision-guided weapons—is well underway. It is no longer possible for the U.S. military, or any military, to win a full-blown war simply by using small numbers of exquisite weapons to dazzle opponents. Weaker countries can match stronger ones through offensive barrages. Offense is currently cheaper than defense: the price of effective interceptors has yet to fall to the level of attack drones and missiles, which means defending against precise mass is extremely costly. After all, militaries cannot endlessly block salvos of $35,000 Shahed-136 attack drones with $4 million Patriot missiles.

To be sure, the war in Iran also shows that advanced technology remains essential. The United States, for example, had great success hitting valuable Iranian sites because it has artificial intelligence systems that can identify and recommend targets faster than ever before. American exquisite weapons are still much harder to stop than cheaper ones, and they are more likely to destroy targets in a single strike. But the world is learning from Iran, building on what Ukraine had already made clear: the future of war lies in part in precise mass. Everyone will now embrace the imperative of producing and using hosts of cheap and disposable weapons systems, changing warfare and the military balance.

In war, it might seem as if wealthy states always benefit from new military capabilities. But the reality is more complicated. If a new system is expensive, it indeed favors rich incumbents. But if it is cheap and demands large organizational changes to adopt, other countries can take advantage just as........

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