Aside from a potentially game-changing American intervention that shapes the fate of Iran’s nuclear program, two factors will help decide the length of the Israel-Iran war: Israel’s reserve of missile interceptors and Iran’s stock of long-range missiles.

Since Iran started retaliating against Israel’s fire last week, Israel’s world-leading air defense system has intercepted most incoming Iranian ballistic missiles, giving the Israeli Air Force more time to strike Iran without incurring major losses at home.

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

Already, Israel’s military has had to conserve its use of interceptors and is giving greater priority to the defense of densely populated areas and strategic infrastructure, according to the officials. Most spoke on the condition of anonymity to speak more freely.

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

“If a missile is supposed to hit refineries in Haifa, it’s clear that it’s more important to intercept that missile than one that will hit the Negev desert,” General Kochav said. Conserving Israel’s interceptors is “a challenge,” he added. “We can make it, but it’s a challenge.”