The U.S. has transferred tens of thousands of its bombs and shells to Israel since Hamas’ Oct. 7 attack.

But it hasn’t given Israel everything it wants. That’s because the U.S. military lacks the capacity to provide some of the weapons Israel requested, according to Gen. CQ Brown, chairman of the Joint Chiefs of Staff.

“We do make recommendations based on what their ask is and how that impacts our readiness if it’s going to come from our stocks,” Brown told reporters during a Defense Writers Group event in March.

Put simply, the U.S. assesses the health of its own inventories before sending weapons abroad. At times, those stocks don’t have any margin — and in some cases, the U.S. is even dipping below minimum inventory requirements, according to congressional staffers and former Pentagon officials.

In addition to Israel, the Biden administration has sent an enormous quantity of materiel to Ukraine since Russia’s 2022 invasion. Meanwhile, the U.S. is gearing up to rush an influx of arms to Taiwan in hopes of deterring a possible Chinese attack on the island, which Beijing considers a rogue province.

The U.S. Defense Department already struggled to maintain robust munitions levels in the decades before the recent wars in the Middle East and Europe. But the shipment of arms to Israel, Ukraine and Taiwan has placed intense pressure on the Pentagon’s inventory, forcing it to make challenging risk management assessments as it tries to move the defense industry from peacetime production to a wartime footing.

“The [Defense Department] is likely drawing down close to that minimum number that they would need for multiple ground scenarios that could happen simultaneously,” Bryan Clark, a senior fellow at the Hudson Institute think tank, told Defense News.

“And depending on how the leadership has decided to manage risk, they may have gone below that two-scenario number already, but they would certainly not go below the number needed for one scenario.”

Total munitions requirement

At a March conference in downtown Washington, the Pentagon’s top weapons buyer opened with a message for America’s adversaries.

“You do not want to go to war with the United States,” said Bill LaPlante, the undersecretary for acquisition and sustainment. “Our stocks are not depleted.”

Perhaps more important was his reason why.

“Every time that we make a decision, we’re looking at our stocks and saying: ‘Can we do this and take the risk?’ ” he said. “If we do, it means we’re OK.”

In short, the Pentagon has a system, and the public should trust it, the argument goes. That system, known as the annual munitions requirements process, has three phases: choosing what to target, how to target it and what to buy.

The first part starts with the Defense Intelligence Agency, which considers the targets America would need to hit if it went to war against another country. The agency sends its list to the relevant combatant commands, which handle U.S. military operations around the globe.

The commands then develop their plans around these targets, and then assign each target to the military services, who study how to best hit each one.

“The services say: ‘What’s my best way to deal with this target?’ ” said Chris Michienzi, a former Pentagon official who spent years working on this process. “ ‘Do I use this airplane with this missile?’ ”

Pentagon officials then use a classified formula to calculate how much of each different munition they need per year, which is known as the total munitions requirement.

Analysts, former defense officials and congressional aides said it’s been difficult to produce enough weapons to execute the Pentagon’s National Defense Strategy around the globe.

“Pre-Ukraine, we had munitions requirements that were in almost every important case — particularly for the Indo-Pacific — not even close to being met,” a Republican congressional staffer told Defense News, speaking on the condition of anonymity due to the sensitivity of the topic. “For the most important [Indo-Pacific] munitions, we haven’t hit the total munitions requirement.”

The shortages are in part symptoms of a chronic issue, said a senior defense official, granted anonymity to discuss the closely held process. The Pentagon has long used munitions as a “bill payer,” neglecting their purchase in favor of platforms like ships or planes in the annual budgets, the official added.

Over time, the low orders led to some companies exiting the market, which in turn reduces the number of businesses that will build those munitions and the speed at which they come off the line.

“There are very few places where we have what you might call surplus stockpiles,” said Mark Cancian, a senior adviser at the Center for Strategic and International Studies think tank. “It’s a question of how much risk do you want to accept in our own war plans. That has been the driver in a lot of the decisions about what to give to the Ukrainians and the Taiwanese.”

For example, the U.S. could use Javelin anti-tank missiles or Tomahawk cruise missiles against at least four major competitors: China, Russia, North Korea and Iran. But the military doesn’t necessarily expect to fight all four adversaries at once and may calculate requirements based on fighting two enemies at a time.

“So you can choose a couple of scenarios and say, ‘Here’s two scenarios that are very stressing,’ and they’re going to form the basis for a number,” Clark said. “For example, the number for the Javelins is probably driven by Russia and North Korea. It depends on the weapon.”

‘Do we have enough?’

But sometimes these projections fall short. In 2016, for instance, the Air Force said it lacked enough munitions — including Hellfire missiles, Joint Direct Attack Munition kits and Small Diameter Bombs — during its campaign against the Islamic State group in Iraq and Syria.

The shortages prompted the Air Force to decline some allies’ requests to buy the in-demand munitions.

The U.S. often serves as a “backstop” for European allies, Clark noted, pointing to NATO’s heavy reliance on American munitions in its 2011 Libya campaign.

“It’s not so much, are we going to have enough weapons to sustain our own capacity for a ground war, because we probably do,” Clark said. “It’s, do we have enough to sustain our own capacity to fight and also support our European allies who may need augmentation because clearly they don’t maintain the magazines to sustain themselves.”

Others interviewed about the munitions requirements process also noted it lags behind real-world events and is closely tied to the Pentagon’s war plans, which usually project short conflicts instead of the reality of longer, protracted wars.

But the U.S. could still quickly run through certain munitions even in a short conflict with a major adversary like China.

A wargame conducted by the Center for a New American Security think tank and the House Committee on the Chinese Communist Party last year found the U.S. would run out of long-range, precision-guided munitions in less than a week in a fight with China over Taiwan. Outgoing committee Chairman Mike Gallagher, R-Wis., subsequently told Defense News that America’s inventory of long-range anti-ship missiles stood at 250 last spring, noting a conflict with China would require at least 1,000.

Since the Israel-Hamas war began in October, the U.S. has also used weapons that could be relevant to an Indo-Pacific battle, like the Standard Missile-6 and Tomahawks, to respond to Houthi attacks on Red Sea shipping lanes off Yemen’s coast.

“Is it a sustainable, long-term strategy to use million-dollar munitions to shoot down drones and loitering munitions that are $10,000, $15,000, $20,000 a piece?” Rep. Rob Wittman, R-Va., asked Gen. Michael Kurilla, the U.S. Central Command leader overseeing forces in the Middle East, during a House hearing in March.

Kurilla stressed the need for the services to create more “cost-effective” counter-drone systems based on directed-energy and laser technology to use against Houthi attacks, instead of launching costly missiles.

The Standard Missile-6 and Tomahawk weapons cost several million dollars per unit. Navy Secretary Carlos Del Toro told Congress in April the service is “approaching $1 billion in munitions” it needs to replenish as a result of its Red Sea operations.

Meanwhile, Israel and Ukraine both need U.S.-supplied air defense systems, including the Patriot, a system Taiwan also uses.

Cancian said there’s “moderate” overlap in the munitions each of the three security partners needs.

“There is some overlap and some risk that one or the other partners is going to have to live with, but there’s also many elements that are not overlapping,” he noted. “Conflict in the Western Pacific is going to be mostly air and naval, whereas what we see in Ukraine is mostly ground.”

Most of the weapons the U.S. has transferred directly to Israel are tens of thousands of air-to-ground munitions to drop on Gaza — bombs Ukraine can’t use as effectively given Russian air superiority.

Ukraine has struggled to bolster its air defenses, partly because an additional $48 billion in security aid was stalled in Congress for more than six months after President Joe Biden submitted his foreign aid request last year. Congress ultimately passed the supplemental spending package in April, which includes $14 billion for Israel and another $4 billion for Taiwan and Indo-Pacific allies, among other non-security initiatives.

Israel receives most of its U.S.-provided weapons through congressionally subsidized arms sales, which allowed the Biden administration to continue arming the nation without the supplemental spending package.

Right now, Taiwan also receives most of its U.S. weapons through arms sales. But constraints on the American industrial base — such as workforce shortages and supply chains hiccups — have contributed to delivery delays for some munitions orders from the island.

Expanding capacity

The Pentagon hopes the foreign aid legislation will allow it to continue large-scale arms transfers to friendly countries. And as the department replenishes systems to those three partners, it hopes the additional munitions demand will pump resources into lagging munitions production lines. A significant chunk of that will go toward increasing domestic munitions capacity in the U.S.

“The further we go along in this and get additional capacity, [the more] the defense-industrial base picks up its pace, then you can actually … take a little bit more risk because you’ve got a capability coming behind,” Brown, the Joint Chiefs chairman, said in March. “The conversation I’ve had with many of our NATO countries is they’re also looking at how to increase their defense-industrial base and capacity.”

But even with the foreign aid legislation, expanding industrial base capacity is no simple task.

“There is such a gap between where the collective West is and where it needs to be in terms of munitions stockpiles,” national security adviser Jake Sullivan told reporters in April. “There is a need for replenishment on all of these systems that extends out years.

“If anything, I believe that our defense industry is still underestimating, rather than overestimating, the need regardless of the precise duration or course of the war in Ukraine.”

Defense Secretary Lloyd Austin told Congress in October that some contractors have required employees to work additional shifts to keep up munitions production rates, highlighting labor shortages in the industrial base.

“What they’ve done in a lot of cases to meet urgent needs is double and triple shifts so that they can, in some cases, crank out munitions and weapons at a much greater speed,” Austin told Congress at the time.

“There are some limitations in terms of how quickly they can do certain things,” he added. “There will continue to be workforce challenges. And when you expand capacity, there’s this issue of the time it takes to build the capacity and make sure the lines are running smoothly.”

A former senior Pentagon official who now works in the defense industry, speaking on the condition of anonymity because the individual was not authorized to talk to the press, told Defense News the Pentagon is generally willing to take more risks on munitions inventory levels than in other areas, expecting that Congress will quickly fund replenishment efforts.

“The mentality in the Pentagon is if I do get in a fight, Congress is going to be real responsive to give me as much money as I need,” the former senior defense official said. “Right now, we’re having a problem replenishing artillery for a war in Europe that we’re not even in.”

“The thing that scares the living crap out of me is right now a large number of that capacity is depending on these supplementals.”

Only the most senior leaders in the Defense Department can adjust munitions inventory requirements, and they rarely do, according to the Republican congressional staffer. The staffer noted that lawmakers may try to address what they consider an excessive focus on short-term conflicts as they draft the annual defense policy bill in the weeks ahead.

The Pentagon in 2022 asked Congress for a critical munitions acquisition fund, which would allow it to buy important weapons before they are transferred and maintain a continuous order of munitions, rather than backfilling them. However, congressional appropriators were cool on the idea, viewing it as a slush fund.

Instead, Congress authorized multiyear contracts for critical munitions to ensure a steady demand signal to industry — a mechanism usually reserved for big-ticket purchases like ships and aircraft. Defense appropriators funded six of the seven multiyear munitions contracts the Pentagon sought for fiscal 2024.

The current senior defense official said the Pentagon intends to submit a revamped proposal for a critical munitions acquisition fund in the coming weeks, calling this version a munitions readiness account.

“As we’ve noticed with Ukraine when we go to stockpile these back into our own stocks, it is two to three years that it’s going to take for us to even replenish what we have provided, even if it’s an upgraded system,” the official said.

Bryant Harris is the Congress reporter for Defense News. He has covered U.S. foreign policy, national security, international affairs and politics in Washington since 2014. He has also written for Foreign Policy, Al-Monitor, Al Jazeera English and IPS News.

Noah Robertson is the Pentagon reporter at Defense News. He previously covered national security for the Christian Science Monitor. He holds a bachelor’s degree in English and government from the College of William & Mary in his hometown of Williamsburg, Virginia.