On Thursday, the acting United States Navy secretary, Hung Cao, was asked by a Senate committee about $14bn in weapons sales to Taiwan that Congress has approved but that President Donald Trump needs to sign off on.

“Right now we’re doing a pause,” said Cao, “in order to make sure we have the munitions we need for Epic Fury – which we have plenty.”

Epic Fury is the name of the US military operation that the Trump administration launched on February 28 against Iran. Trump’s Secretary of State Marco Rubio has since stated that Epic Fury was over, so Cao’s comments on needing munitions for the operation appeared to contradict the US’s top diplomat.

But the Navy secretary’s comments on needing munitions for the Iran war also hit at another contradiction between the administration’s claims and the facts.

While Cao was adamant that the pause in supplies to Taiwan was not due to a critical shortage of weapons – echoing other officials in the Trump administration – there’s growing evidence that the US is running through its munitions and missiles in the war on Iran much faster than they can be replenished.

That strain was captured by Cao’s own comments. “We’re just making sure we have everything, then the foreign military sales will continue when the administration deems necessary,” he told senators.

On Thursday, The Washington Post revealed that the US used more of its advanced missile-defence interceptors to defend Israel than even Israel itself during the 40 days of the Iran war, before the ceasefire came into effect on April 8.

The report found that the US launched more than 200 Terminal High Altitude Area Defenses (THAAD) interceptors – which equate to about half its total inventory – and more than 100 Standard Missile-3 and Standard Missile-6 interceptors. By contrast, Israel fired fewer than 100 Arrow interceptors and about 90 David’s Sling interceptors.

While US officials publicly project confidence in their stockpiles, analysts say the diminishing munitions could be factoring in Washington’s calculations in resuming its war on Tehran.

Risk lies in future wars

Following The Washington Post’s report, the Pentagon and Israel both defended their joint strategy of interceptor deployments, with the US defence body downplaying concerns of dwindling weapons.

Ballistic missile interceptors are “just one tool” in the US air defence network, and both Israel and the US “equitably” shouldered the burden of Operation Epic Fury, Sean Parnell, the Pentagon’s chief spokesman, told the US outlet.

Even as early as the first week of the war, Trump shrugged off concerns about US stocks of some critical missiles running low.

But in late April, when the ceasefire had reduced the daily use of missiles and interceptors, the Center for Strategic and International Studies (CSIS), a Washington, DC think tank, warned that US forces had heavily used seven critical ammunitions, and for four of them expended more than half of the pre-war inventory. They included the THAAD interceptors, Patriot missiles and the SM-3 and SM-6 ship-based surface-to-air missiles used to intercept ballistic missiles.

By April 21, the CSIS report said, the US had also used up more than 1,000 of its estimated 3,100 Tomahawk missiles.

“Rebuilding to pre-war levels for the seven munitions will take from one to four years as missiles in the pipeline are delivered,” the report stated.

To be sure, Felix Arteaga, a defence and security fellow at Madrid’s Elcano Royal Institute, said that unless a conflict with China erupts over Taiwan from today, the US is largely prepared – for now.

“They will have prepared because they have made calculations for planning – alternative planning, contingency planning, [and] emergency planning,” Arteaga told Al Jazeera.

But Omar Ashour, a professor of security and military studies at the Doha Institute for Graduate Studies in Qatar, said that while the Iran war did not empty the US arsenal of weapons, it burned through some of the most important and strategically valuable layers of this arsenal.

“It’s not tactical exhaustion, it’s just a strategic inventory shock if you wish, because that depletion will affect other theatres [of war],” Ashour told Al Jazeera.

He added that while expenditure on weapons like Tomahawks was serious, the missile defence depletion was a sharper strategic problem for the US.

While CSIS said the US had enough missiles to continue fighting the Iran war in “any plausible scenario”, the risk “which will persist for many years, lies in future wars”, it said.

The weapons factor in Washington’s Iran war calculations

While Trump has insisted he has called off military attacks in the past few weeks on Iran at the request of US allies in the Gulf, Ashour said dwindling munitions are indeed part of his calculation on this decision.

“Stockpiles are now part of the escalation calculus,” said Ashour.

“The US can restart striking, but every renewed wave has an opportunity cost … The question is no longer ‘Can we strike?’, but, ‘What strategic magazine are we consuming to strike and how would that affect the other theatres [of war]?’”

The Washington Post also reported that Israel’s persistent pressure to restart the war has “irritated some US officials, particularly given the strain that renewed fighting would impose on the Pentagon’s munitions supply”.

A CSIS report in April found that Iran’s missiles and drones during the war destroyed US military equipment worth between $2.3bn and $2.8bn, an additional factor in its diminished supply.

And despite Trump’s claims that Iran’s missile arsenal has been “mostly decimated”, US intelligence assessments indicate Tehran still retains about 70 percent of its pre-war stockpile.

So while the Iran war did not prove that the US is weak, it proved that Iran is resilient, Ashour said.

“[Iran is] capable of multi-domain resistance and capable of withstanding 21,000 strikes and … sustained decapitation without collapsing. Not many states, not many regimes can claim that.”

The US does not have an ‘unlimited magazine depth’

According to the April CSIS report on US munitions, building stockpiles of weapons for future wars will take years. Even rebuilding to pre-war levels for the seven critical munitions the report cited “will take from one to four years as missiles in the pipeline are delivered”.

Long lead times, supply chain bottlenecks, workforce constraints and material availability are challenges impacting manufacturing at speed, wrote Albert Vidal Ribe, an analyst at the International Institute for Strategic Studies.

This is creating challenges not only for the US, but also for its Gulf allies, many of which relied on US-made and US-supplied defence systems to fend off retaliatory Iranian attacks, Arteaga said.

While confidence in those systems remains high because they largely proved effective, Gulf states also understand that dwindling US munition stockpiles could leave them more vulnerable if the war resumes, he added.

Trump’s second administration has attempted to expand production, but growth has been slower than hoped for due to the earlier COVID-19 pandemic-related supply-chain issues, and higher inflation and energy costs staving off extra investment, according to Ribe.

Arteaga stated it plainly: “American factories are increasing production lines … but are not delivering in the same quantity they [US forces] are consuming.”

Trump has repeatedly voiced frustration over the pace at which US weapons manufacturers are producing munitions.

The shortage of US interceptors, meanwhile, has alarmed US allies in Asia, particularly Japan and South Korea, which rely on Washington as “a deterrent to potential threats from North Korea and China”, the Post reported.

“The Iran war is exactly the kind of contingency that would worry any Indo-Pacific strategic planner. Why? Because that’s the regional campaign consuming the same types of precision-strike and missile defence inventory needed for a China fight,” said Ashour.

Diminished inventories will also affect the US supply of Patriot, THAAD, and Precision Strike Missiles (PrSMs) to Ukraine, CSIS has said.

The war in Iran, therefore, has exposed the US’s structural imbalance, said Ashour.

“The US military can generate high-intensity fires faster than the defence industrial base can regenerate them.”

It is not just money, but also production lines, motors, skilled labour, rare earth materials – all issues that can’t be resolved by “throwing money at them”, he added.

“The US still has global reach, but it does not have unlimited magazine depth.”