The mushroom clouds over Hiroshima and Nagasaki have remained unreplicated for 75 years in part because the U.S. and Soviet Union — after peering over the ledge into nuclear armageddon — began to negotiate.

Why it matters: The arms control era that began after the Cuban Missile Crisis may now be coming to a close. The next phase could be a nuclear free-for-all.

Driving the news: New START, the last major pact constraining the U.S. and Russia — which together hold 91% of the world's nuclear warheads — is due to expire two weeks into the next presidential term, on Feb. 5.

• President Trump has insisted on a new, larger deal that also constrains China — which has a fast-growing but far smaller program.
• Beijing balked at American pressure to join recent U.S.-Russia nuclear talks in Vienna.
• Trump has recently held a flurry of phone calls with Vladimir Putin, and he suggested last week that he might support a deal involving only the U.S. and Russia.
• His top aides continue to insist on Chinese participation and to resist Putin's proposal for an unconditional extension.

Where things stand: Joe Biden today reiterated his commitment to extending New START if elected (he'd have 15 days to exchange diplomatic notes with Russia). Its fate would be far more uncertain in a second Trump term.

What they're saying: Rose Gottemoeller, the chief U.S. negotiator for New START under Barack Obama, says China won't rush to the negotiating table before February.

• "It’s not rational, it’s not logical and they have absolutely no incentive to do so," she told Axios.
• Meanwhile, "the Russians have just finished the modernization of their nuclear arsenal," and they have "hot production lines."
• “We could soon be facing a Russia who is far superior to us in nuclear numbers," she says, if the deal lapses.

Tim Morrison, who held both the arms control and Russia portfolios on Trump’s National Security Council, says those demanding a clean extension of New START might as well enter a car dealership and declare: "I am not leaving here until you sell me a car, no matter the price."

• China and Russia fear Washington's deep pockets and the prospect of an unconstrained American arsenal, he says. That offers leverage.
• America, meanwhile, has reason to fear Russia's fast-advancing capabilities (not covered by New START) and China's burgeoning nuclear buildup.
• That explains the administration's emphasis on a new deal and its willingness to take the old one to the brink. “I am simply not in the panic that some people are to extend the treaty now," Morrison says.

The big picture: The expiration of New START — paired with Trump's withdrawal from the Open Skies and Intermediate-Range Nuclear Forces treaties (the latter due to Russian violations) — would signal the end of arms control as we know it.

• If the arms control infrastructure built up during the Cold War collapses, it may be impossible to reassemble.
• Morrison, now a senior fellow at the Hudson Institute, acknowledges that. But he points the finger at Moscow: “If arms control is dying, it’s because Russia killed it."

Even if New START survives, we are heading into a new, more uncertain era.

Why it matters: As communications channels break down and new technologies are built up, "the possibilities of a miscalculation are unfortunately higher than they have been in a long, long time," Ernest Moniz, the nuclear physicist and former energy secretary, tells Axios' Bryan Walsh.

What to watch: Emerging technologies like cyber warfare and AI portend an uncertain future.

• Meanwhile, arsenals of smaller-yield nukes are growing, including among newer nuclear powers like India and Pakistan. "They were built for battlefield use and so seem to lower the threshold for use," Gottemoeller says.
• North Korea has entered the nuclear club, and others could follow. Those include Iran and Saudi Arabia, which is working with China to increase its capacity to generate nuclear fuel, the NYT reports.
• With Russia boasting of "unstoppable" weapons and China working to narrow the nuclear gap, America is undertaking a modernization process that could cost $1.7 trillion over 25 years, Jessica Mathews writes in the New York Review of Books.

The bottom line: "Yet the public isn't scared," Mathews writes. "Decades of fearing a nuclear war that didn’t happen may have induced an unwarranted complacency that this threat belongs to the past."

Go deeper: How new tech raises the risk of nuclear war

There are roughly 13,355 nuclear weapons in the world, with 91% of them belonging to Russia (6,370) or the U.S. (5,800), according to estimates from the Bulletin of Atomic Scientists.

• China’s stockpile of around 290 warheads is “likely to grow further over the next decade” and put it firmly in the third spot among the world’s nuclear powers, according to analysts Hans Kristensen and Matt Korda.
• France (300) and the U.K. (215) both have significant stockpiles, as do rival nuclear powers Pakistan (150) and India (130).
• Israel has never confirmed or denied possessing nuclear weapons, but it's believed to have secured the bomb in 1966 or 1967 and possess around 80 warheads.
• North Korea, meanwhile, has embraced its status as a new nuclear power since conducting its first test in 2006. In 2019, the analysts put its stockpile at 20–30 warheads.

Timeline: The U.S. was first to the bomb, conducting its Trinity test 75 years ago last month.

• The Soviet Union conducted its first nuclear test in 1949, far earlier than U.S. officials had expected. President Harry Truman attempted to downplay public fears, saying, “The eventual development of this new force by other nations was to be expected.”
• The nuclear club grew as the U.K. (1952) and later France (1960) conducted tests.
• The Chinese, after initial Soviet help, tested a nuclear weapon in 1964. A wary U.S. considered plans to sabotage China’s nuclear program.
• The 1968 Nuclear Non-Proliferation Treaty has helped limit the size of the club, though India (1974), Pakistan (1998) and North Korea (2006) brought the membership to nine, including Israel.
• South Africa is the only country to develop nuclear weapons and then dismantle them, in 1989.

Shortly after midnight on Aug. 6, 1945, "the crew of the B-29 named Enola Gay is told what weapon they will be carrying," and warned it "might crack the Earth’s crust," Jonathan Holmes writes for the Telegraph in a beautiful and terrifying history of the day.

Excerpts:

02.45 "The Enola Gay takes off. ... "the crew still do not know where they are heading."

07.29 “The hour was early; the morning still, warm, and beautiful,” Dr. Michihiko Hachiya, an air raid warden, noted in his diary. “Shimmering leaves, reflecting sunlight from a cloudless sky, made a pleasant contrast with shadows in my garden as I gazed absently through wide-flung doors opening to the south.”

08.15 "The bomb bay doors open. It is a sunny day."

• "Naked survivors, the patterns of their clothes burned into their backs, wander dazed through the inferno, driven by an unbearable thirst. ... A black rain falls from the sky. Hiroshima is now, in a word repeated over and over again by survivors, 'Hell.'"
• "The strike soon developed into an information war. Initial official reports to the Japanese people were that the US had deployed a new, ‘special bomb’, but the damage was minimal. ... There were rumours that Japan had secretly developed its own ‘special’ bombs, and had destroyed Los Angeles."

Breaking it down: Estimates of the death toll in Hiroshima range from 70,000 to 140,000, with about half as many in Nagasaki, per the Bulletin of Atomic Scientists.

• The bomb that struck Hiroshima, Little Boy, had about 50x the force of Tuesday's non-nuclear blast in Beirut, Bryan notes.
• The largest weapon in America's current nuclear arsenal, meanwhile, has 80x the explosive power of Little Boy.

Go deeper: Understanding the true scale of a nuclear bomb

French President Emmanuel Macron walked through the blast-damaged streets of Beirut on Thursday, swarmed by people chanting for the fall of Lebanon's government and pleading for international aid.

Why it matters: Lebanon is at a breaking point. Its economy was collapsing and its government hardly functioning — all before a massive explosion destroyed swathes of the capital city, including its vital port.

• Macron was the first national leader, foreign or domestic, to visit Beirut's hard-hit neighborhoods.

What he's saying: "I will propose a new political pact in Lebanon, and I will be back in September, and if they can't do it, I'll take my political responsibility," he told a crowd, per AFP's Mohamad Ali Harissi.

Between the lines: That's quite a statement from the leader of a former colonial power.

• It shows both Macron's willingness to position himself as a — perhaps the — global leader, and the utter lack of faith Lebanese people have in their own government to rebuild.

On the ground: Just two days after the explosion, crowds swarmed around Macron, desperate to be heard. "Please help us. What are you doing to help us?" one man could be heard shouting through tears.

Where things stand: The death toll in Beirut has surpassed 135, with at least 5,000 injured, in what appears to have been an accidental explosion.

On Sept. 23, 2013, a Russian-owned, Moldovan-flagged ship departed Georgia en route to Mozambique bearing 2,750 tons of ammonium nitrate, a material used in fertilizer as well as explosives.

Why it matters: The Rhosus made an unscheduled stop in Beirut, apparently due to engine problems. The ammonium nitrate never left the port, but destroyed it nearly seven years later, along with much of the city.

How it happened: Authorities did not allow the Rhosus to travel on from Beirut, either because of its technical issues or a lack of proper documentation. Attempts by authorities and creditors to reach the ship’s owners were unsuccessful. Crew members were left stranded. So was the cargo.

In July 2014, under the headline "Crew kept hostages on a floating bomb," ship-tracking site FleetMon reported that the ship's owner — a Russian-born Cyprus resident named Gregushkin Igor — had abandoned it with four crew members still stranded on board.

• The crew was eventually freed, and the ammonium nitrate was moved to Hangar 12, a storage facility at the port.
• "The vessel and cargo remain to date in port awaiting auctioning and/or proper disposal," lawyers representing the crew wrote in 2015.

Between 2014 and 2017, port officials wrote to Lebanese courts at least six times seeking guidance on what to do with the ammonium nitrate, the NYT reports.

• "As recently as six months ago, officials inspecting the consignment warned that if not moved it would 'blow up all of Beirut,'" according to The Guardian.

1. Abrams to replace Hook as Iran envoy
2. Global travel warning lifted
3. TikTok faces bans around the world
4. State Dept. inspector general out after 3 months
5. U.S. politicians exploit foreign election interference
6. China: U.S. "endangering peace" with high-level Taiwan visit
7. UN: Global school closures are "generational catastrophe"

Quoted:

“It is very difficult, but we’re doing this to ensure the health of Bolivians, especially our children.”