The one-ton Mark 84 bomb was designed shortly after World War II. Adding guidance kits has kept it in use for more than 70 years.

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When President Biden threatened to pause some weapons shipments to Israel if it invaded the southern Gaza city of Rafah, the devastating effects of one weapon were of particular concern to him.

“Civilians have been killed in Gaza as a consequence of those bombs,” Mr. Biden said in remarks to CNN this week.

He was referring to U.S.-made 2,000-pound aerial weapons, the largest in the Pentagon’s Mark 80 series of bombs.

In the military’s banal lexicon, the Mark 80s are “general purpose” bombs, meaning that they can be used on almost any target the military typically expects to encounter in war. In addition to the 2,000-pound Mk-84, they also come in 250-pound, 500-pound and 1,000-pound versions — the Mk-81, Mk-82 and Mk-83.

The president has already delayed a shipment to Israel of 3,500 bombs in the Mark 80 series that he feared could be used in a major assault on Rafah, where more than one million Palestinians have taken refuge.

A New York Times investigation in December found that American 2,000-pound bombs were responsible for some of the worst attacks on Palestinian civilians since the war in Gaza began after Hamas attacked Israel on Oct. 7.

According to a U.S. Army office that manages ammunition for the Pentagon, the ideal targets for weapons of that size are “buildings, rail yards and lines of communication.”

However, Defense Department data indicates that U.S. warplanes typically use far less powerful munitions for supporting ground troops engaged with enemy fighters.

The explosive warheads of these bombs have changed little since the U.S. Navy created them shortly after World War II, but the Pentagon has kept them in service by developing new parts and pieces that can be attached for a variety of purposes.

About 40 percent of each one’s weight is composed of a high explosive mixture; the rest comes from its steel case. When detonated, the bomb’s smooth skin shatters into razor-sharp fragments that can shred human bodies and unarmored vehicles alike.

Course guides used in teaching American troops how to call in airstrikes state that anyone within 115 feet of a 250-pound bomb’s impact has a 10 percent chance of being incapacitated or killed. That lethal radius jumps to nearly 600 feet for a one-ton version that explodes just above the ground.

For a time, the United States held a monopoly on these bombs. But now Mark 80s are made and sold by a number of countries, including Australia, Brazil, Canada, France, India, Italy, Pakistan, Spain, Switzerland, Turkey and the United Arab Emirates.

Israel makes its own versions, but export data suggests that the country purchases most of its bombs from the United States through an annual $3.5 billion grant of American taxpayer money.

How have the bombs evolved?

Classified through much of the 1950s, the Mark 80 came fully into public view during the Vietnam War.

Most Mark 80s dropped over Vietnam, Cambodia and Laos from 1965 to 1973 were unguided weapons that cost a few hundred dollars each. Under the best of conditions, about half of them could be expected to land within 400 feet of their target.

When they missed, whether because of pilot error or winds pushing them around after being dropped, they sometimes killed American troops in large numbers in addition to killing civilians.

The use of radar signals to better determine the right place to drop these unguided bombs sometimes failed spectacularly, such as one incident when five jets flying in bad weather mistakenly dropped 34 Mark 82 500-pound bombs on the American air base in Da Nang.

But in the late 1960s, Texas Instruments developed a kit called Paveway that gave the Mark 80 far greater accuracy by adding parts to the bomb’s nose and tail that allowed the bomb to steer itself to a target using lasers shined from warplanes above. That shrank the average miss distance to about 10 feet. Because of their high cost, though, Paveways made up only a tiny fraction of the bombs dropped by the U.S. Air Force during the Vietnam War.

These weapons were commonly called “smart bombs” during the Persian Gulf War in 1991, and the term has endured to describe a host of guided weapons fielded in the decades since.

But the laser-guided weapons often failed in bad weather and sandstorms, leading military officials to develop a new guidance kit for the Mark 80 in the early 1990s. Called JDAM — for Joint Direct Attack Munition — they cost half as much as Paveway and used radio signals from the military’s nascent constellation of GPS satellites in outer space to guide them. They can generally hit within 30 feet of their targets.

How often are 2,000-pound bombs used?

For American forces, not that often.

During the Vietnam War, Air Force warplanes dropped more Mark 82s than all other kinds of aerial weapons combined, including cluster bombs — and usually reserved Mark 84s for destroying large buildings or infrastructure like bridges. In the decades since, the Mark 82 has remained the most commonly used warhead by Americans in combat, especially when combined with a Paveway or JDAM guidance kit.

By comparison, Israel reaches for its 2,000-pound bombs far more often.

In the first two weeks of the war, roughly 90 percent of the munitions Israel dropped in Gaza were satellite-guided bombs of 1,000 to 2,000 pounds, according to a senior U.S. military official. The rest were 250-pound small-diameter bombs.

Israel also uses a slightly different kind of 2,000-pound bomb called the BLU-109 that can penetrate underground to reach buried targets like Hamas tunnels. Like all so-called bunker-busters, most of that weapon’s weight comes from a much thicker steel case than general-purpose weapons, and it explodes with the force of just 525 pounds of TNT — far closer to the power of the 1,000-pound Mark 83.

Are there even larger bombs?

The United States makes very few conventional bombs larger than 2,000 pounds. Israel has acquired one of them, an even thicker-cased 5,000-pound bomb built for attacking targets deeper underground.

Israel purchased 50 such bombs from the United States in 2015. Each carries the equivalent of just 625 pounds of TNT.

The other two weapons have never been sold or provided to allies.

One is a 21,000-pound bomb called the Massive Ordnance Air Blast, or MOAB, which explodes just above the ground with the force of 18,700 pounds of TNT and can only be dropped from cargo planes. It was used once in Afghanistan in 2017, in what is the sole publicly acknowledged use of that weapon in combat.

The service also has a 30,000-pound bomb called the Massive Ordnance Penetrator capable of punching even farther underground before exploding, but it can only be carried by the B-2 stealth bomber. It explodes with the force of 5,600 pounds of TNT.

What’s the opposition to Israel’s use of the Mark 84?

Many politicians and activists say 2,000-pound bombs are too powerful to be used responsibly in Gaza, a densely populated enclave.

“The U.S. cannot beg Netanyahu to stop bombing civilians one day and the next send him thousands more 2,000 lb. bombs that can level entire city blocks,” Senator Bernie Sanders of Vermont posted on social media on March 29, referring to the Israeli prime minister, Benjamin Netanyahu. “This is obscene,” he added. “We must end our complicity: No more bombs to Israel.”

In May 2021, Mr. Sanders, an independent who caucuses with the Democrats, attempted to block a $735 million sale of American bombs to Israel for similar reasons.

Israel has used these weapons before. Israel relied on Mark 80s during another “all-out war” against Hamas in 2008 and used them again in 2021 to destroy a building in Gaza City that housed the offices of The Associated Press, Al Jazeera and other news media organizations.

Israel’s Ministry of Defense did not respond to multiple calls and emails asking for comment on transfers of American-made bombs, including questions about the provision of Mark 84s.

Excerpts: New york times