In addition to an American intervention that could shape the fate of Iran's nuclear program, two factors will help decide the length of the war between Israel and Iran: Israel's missile interceptor stockpile and Iran's long-range missile stockpile.

Since Iran began retaliating against Israeli strikes last week, Israel's air defense system — considered the most advanced in the world — has intercepted most of the Iranian ballistic missiles, giving the Israeli Air Force more time to strike targets in Iran without suffering major losses at home.

Now, as the war drags on, Israel is deploying interceptors faster than it can produce them. That has raised questions within the Israeli security establishment about whether the country will run out of air defense missiles before Iran exhausts its ballistic missile arsenal, according to eight current and former officials.

According to the authorities interviewed, the Israeli army has already had to start rationing the use of interceptors, prioritizing the defense of densely populated areas and strategic infrastructure. Most of the interviewees spoke on condition of anonymity to be able to express themselves more freely.

“Interceptors are not grains of rice,” said Brigadier General Ran Kochav, who commanded Israel’s air defense system until 2021 and is still in reserve. “The number is finite.”

“If a missile is going to hit the Haifa refineries, it is clearly more important to intercept it than one that is going to fall in the Negev desert,” General Kochav said. Rationing Israeli interceptors is “a challenge,” he added. “We can do it, but it is a challenge.”

Asked about the limits of its interceptor arsenal, the Israeli army responded in a brief statement that it is “prepared and ready to deal with any scenario, operating defensively and offensively to eliminate threats against Israeli civilians.”

At the start of the war, some Israeli officials estimated that Iran had about 2,000 ballistic missiles. According to Israeli officials, between a third and a half of that arsenal has already been used — either because Iran has launched them at Israel or because Israel has attacked the depots where they were stored. Iran has fired far fewer missiles in its most recent attacks, possibly aware of the risk of running out of ammunition. Iran’s mission to the United Nations did not respond to a request for comment.

At the same time, Israel is also rapidly running down its interceptors. As of Wednesday morning, Iran had fired about 400 missiles; about 40 of them managed to evade Israel’s air defense system and hit Israeli neighborhoods, the Israel Defense Forces said. The remaining 360 were intercepted or tracked until they fell into uninhabited areas or the sea, the military said. Some of the Iranian missiles may have been hit more than once, and the total number of interceptors used is unclear.

No Israeli official has revealed how many interceptors remain at the country's disposal. Disclosing such a sensitive secret could give Iran a military advantage.

The response will affect Israel’s ability to sustain a long-term war of attrition. The nature of the war will be decided in part by whether President Trump decides to join Israel in attacking Iran’s nuclear enrichment facility at Fordo in northern Iran, or whether Iran decides to abandon its enrichment program to prevent such intervention.

But the outcome of the war will also depend on how long both sides can withstand the damage to their economies, as well as the impact on national morale caused by the rising civilian death toll.

Israel has at least seven different types of air defense systems. Most of these systems are automated and use radar to detect enemy missiles and provide officers with suggestions on how to intercept them. In some cases, the military has only seconds to react to short-range fire, while in others, it takes minutes to decide how to respond to long-range attacks. In some situations, the automated systems provide no recommendations at all, leaving the decision entirely in the hands of officers, General Kochav explained.

The Arrow system intercepts long-range missiles at high altitudes; the David’s Sling system intercepts them at lower altitudes; while the Iron Dome neutralizes short-range rockets — usually fired from the Gaza Strip — or missile fragments already intercepted by other defense systems.

The United States has provided at least two additional defense systems, some of which are fired from ships in the Mediterranean Sea. Israel is also testing a new, relatively untested laser beam technology. Finally, fighter jets are being mobilized to shoot down slower-moving drones.

Some Israelis believe it is time to end the war before the country’s defenses are put to a more severe test. At least 24 civilians have already been killed in the Iranian strikes, and more than 800 have been wounded. Strategic infrastructure, including oil refineries in northern Israel, has been hit, as have civilian homes. A hospital in southern Israel was hit early Thursday.

Already high by Israeli standards, the death toll could rise dramatically if the military is forced to restrict the overall use of interceptors to ensure long-term protection of a few strategic sites — such as the Dimona nuclear reactor in southern Israel or the military headquarters in Tel Aviv.

“Now that Israel has managed to hit most of the nuclear targets in Iran, it has a two- or three-day window to declare victory and end the war,” said Zohar Palti, a former senior official in Mossad, Israel’s intelligence agency.

“When planning Israel’s defense for future wars, no one imagined a scenario in which we would be fighting on so many fronts and defending the country against so many waves of ballistic missiles,” said Palti, who for years has been involved in Israel’s defense planning.

Others, however, are confident that Israel can solve the problem by destroying most of Iran’s missile launchers, preventing Iranian forces from using their remaining stockpiles. According to two Israeli officials, Iran has fixed and mobile launchers spread across its territory. Some of its missiles are stored in underground facilities that are harder to destroy, while others are stored in above-ground storage.

The Israel Defense Forces says it has already destroyed more than a third of those launchers. Officials and experts say that has limited the number of missiles Iran can fire in a single strike. U.S. officials say Israel’s strikes on the launchers have decimated Iran’s ability to launch its missiles and weakened its ability to conduct large-scale offensives.

“The real issue is the number of launchers, rather than the number of missiles,” said Asaf Cohen, a former Israeli commander who led Iran’s department of military intelligence.

“The more of them that are hit, the harder it will be for Iran to launch mass offensives,” Cohen added. “If they realize they have a launch capability problem, they will switch tactics to continuous harassment: one or two missiles at a time, targeting two different areas at the same time.”