As Iran reduces the breakout time it needs to build atomic weapons, President Biden is asking Congress to approve an upgrade of the B61-12 nuclear gravity bomb. The new B61-13 would be able to destroy underground nuclear sites if all else fails.

Mr. Biden told the Israeli prime minister at the time, Yair Lapid, in August last year that America would never allow Iran develop the atomic weapons, knowing they would use them to make half a century of “Death to Israel” and “Death to America” threats radioactive reality.

In the joint statement with Israel the month before meeting with Mr. Lapid, the president said America stood ready to use “all elements of its national power” to prevent Iran from joining the nuclear club, including force as a “last resort.”

Tehran began counter measures to preserve its twisted version of the Manhattan Project. According to analysis of satellite images by the James Martin Centre for Nonproliferation Studies quoted by the Economist in May, Iran is building hardened facilities in the Zagros Mountains near its Natanz enrichment site.

Buried 260 to 330 feet deep, the Economist reported that the Zagros sites are “invulnerable even to America’s most powerful bunker-busting bomb.” Even the existing B61-12 might not be able to put it out of commission with a single strike.

With Iran funding Hamas in its war against Israel and threatening America, Mr. Biden risks being all bark and no bite on deterring their nuclear program. He’d be powerless to prevent Iran’s supreme ruler, Ali Khamenei, from repeating J. Robert Oppenheimer’s quote from Hindu scripture, “I am become Death, the destroyer of worlds.”

So, on Friday, the Department of Defense asked Congress to approve and support development of the B61-13. The weapon would be, if not a last resort, then the second-to-last resort to stop the mullahs from unleashing the apocalypse.

“The B61-13,” the Pentagon wrote in a statement, “will strengthen deterrence of adversaries and assurance of allies and partners by providing the president with additional options against certain harder and large-area military targets.”

The existing version of the B61 is the only known earth penetrator in America’s arsenal. It delivers a ground shock of 360 kilotons, 24 times more powerful than the 15-kiloton bomb dropped on Hiroshima, Japan, in 1945.

Unlike the weapons used to end World War II, a gravity bomb’s destructive force is pointed downwards. To do the same damage to underground facilities would require a surface burst 15 to 25 times larger than a B61 payload according to the Federation of American Scientists.

The Federation wrote after Friday’s announcement that the “case for the B61-13 is strange,” which may complicate congressional approval. “Military officials have explained countless times that the B61-12 would be able to cover all gravity missions with less collateral damage than large-yield bombs.”

“Detonating the weapon closer to the target,” the Federation wrote, “will increase the probability that the target is destroyed, and a very hard facility could hypothetically be destroyed with one B61-13 instead of two B61-12s.”

Mr. Biden’s goal in producing the B61-12s last year was to allow America to slash the total number of gravity bombs in half by increasing accuracy with the addition of a tail kit and replacement of its parachute. Reducing the amount of fissile material, they argued, would also reduce the danger of payloads being stolen.

Of the Obama administration’s plans to upgrade the B61s in 2015, the Federation wrote that when the weapons were first developed in the 1990s, it was “despite concerns that it could increase the likelihood of use.”

The geopolitical situation in 2023 has made those concerns seem prophetic, but it’s Iran’s belligerence that may increase the chance that a future president will have to deploy a version of the gravity bomb in the nation’s defense.

As in World War II, America will have to weigh the international blowback for using nuclear weapons against the prospect of greater bloodshed without it. Having the B61-13 may be enough to make Tehran’s ruling mullahs think twice by giving Mr. Biden the power to back up his words and ensure that Iran will never gain the power to, in the words of atomic pioneer J. Robert Oppenheimer, “Become Death.”