(WASHINGTON) — President Joe Biden has announced that the United States is “surging” additional military assistance to Israel, including missile interceptors “to replenish Iron Dome” as the air defense system struggles to fend off thousands of Hamas rockets.

“We’re going to make sure that Israel does not run out of these critical assets to defend its cities and its citizens,” Biden said in remarks Tuesday at the White House.

The Hamas assault on Israel began Saturday morning when the militant group that controls the Gaza Strip launched thousands of rockets at targets inside Israel, followed by a ground assault by an estimated 1,000 Hamas militants who attacked Israeli towns located close to Gaza.

The initial wave of rocket attacks by Hamas was larger than any seen in recent memory in repeated military conflicts between Israel and Hamas.

The rocket attacks appeared to have at times overwhelmed Israel’s Iron Dome system that, until now, has been largely successful in intercepting large numbers of rockets aimed at Israel fired by Hamas or Hezbollah, the militant group based in southern Lebanon.

The combined rocket and ground attacks by Hamas have already killed more than 900 Israelis, according to Israeli health officials, and the subsequent Israeli retaliatory airstrikes in Gaza have killed more than 900 Palestinians, according to the Palestinian Health Authority.

Here’s what to know about Israel’s Iron Dome:

What is the Iron Dome?

Like many modern air defense systems, an Iron Dome battery uses a sophisticated radar system to track inbound missiles that are then intercepted by Tamir missiles fired from multiple launchers attached to the radar system.

Each Iron Dome battery consists of three to four launchers that can each carry up to 20 Tamir interceptor missiles.

The system can bring down rockets fired from a range of two to 40 miles away.

The mobile air defense system first became operational in 2011 and since then has had a very high success rate in intercepting rockets targeting Israel.

The Israeli Defense Force (IDF) has disclosed that in May, when Islamic Jihad launched a barrage of rockets towards Israel, 95.6 % of the Iron Dome interceptors launched by the system successfully destroyed incoming rockets.

“Of the 1,469 Gazan rockets which were launched, only 1,139 rockets managed to cross into Israeli territory,” the IDF said on its website. “The Iron Dome successfully intercepted 437 rockets, which is a 95.6% success rate of all interceptions.”

The IDF’s information highlights the reality that an interceptor missile is not fired at every rocket that makes it into Israeli territory. Instead, based on tracking data, the system fires interceptors only if an incoming rocket poses a threat to a populated area. If not deemed to pose a threat, then the incoming rocket will land in an unpopulated area.

The Iron Dome is typically located around cities or smaller populated areas that have been targeted in the past by incoming rockets.

The IDF has said that Hamas fired close to 2,500 rockets in the initial first wave while Hamas claims it fired 5,000. Regardless, that number of incoming rockets is significantly larger than anything the Iron Dome has previously tracked or intercepted in a short amount of time.

Part of Hamas’ strategy in launching so many rockets into Israel may have been intended to take advantage of the Iron Dome’s human element. While the Iron Dome could likely track every incoming rocket and launch all available interceptors, those rocket batteries would eventually run out of missiles and have to be reloaded with new batches.

That scenario would mean there would be a gap in coverage while the new missiles are added.

Where are the extra Iron Dome missiles coming from?

After President Biden announced that the U.S. is surging assistance, national security adviser Jake Sullivan told reporters at the White House that the replenishment of some of the missile interceptors launched by the Iron Dome had already taken place.

“Some of the interceptors have come out of stocks that the U.S. had in country (Israel),” said Sullivan.

“So, those have gone over to the Israelis in short order and then we will be flowing in additional Iron Dome interceptors so that they have the capabilities they need to sustain their Iron Dome defense systems,” he added.

In August, the U.S. Marine Corps announced that it would purchase three Iron Dome batteries consisting of 44 launchers and 1,840 Tamir interceptor missiles.

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