Constraints on nuclear proliferation have lapsed or been loosened in recent years. How great is the danger? Here's everything you need to know:

Who has nuclear weapons?
The vast majority — some 91 percent — of the world's 13,400 nuclear weapons are owned by the U.S. and Russia, which each have the power to render Earth an uninhabitable nuclear wasteland. The other early developers of nuclear arsenals were the U.K., China, and France. In an attempt to prevent further spread, the Non-Proliferation Treaty (NPT) was adopted in 1970, pledging those five powers to eventually disarm in return for other states promising not to pursue the bomb. But more than 50 years later, all four of the countries that aren't party to the treaty — India, Pakistan, Israel, and North Korea — have nuclear arsenals (although Israel has never confirmed it), and at least one signatory, Iran, has taken steps to build its own. Another treaty, the Treaty on the Prohibition of Nuclear Weapons, just came into force in January, but none of the nuclear states signed it. Though public concern about nuclear war has faded since countries became preoccupied with terrorism, climate change, and now, viral pandemics, the threat remains very real. Potential triggers of nuclear conflict include India's border disputes with both Pakistan and China, Iran's threats to destroy Israel, Israel's pledge to prevent Iran from getting nukes, China's designs on Taiwan, and North Korea's threat to South Korea.

What about arms control treaties?
Few remain. During the Reagan era, the U.S. and the Soviet Union agreed to slash their nuclear arsenals, but most arms control treaties since then have lapsed. The Bush administration pulled out of the Anti-Ballistic Missile Treaty in 2002, which sparked an arms race in missile-defense systems, and President Trump yanked the U.S. out of the Intermediate-Range Nuclear Forces Treaty in 2019, saying that Russia had violated it. So the only remaining arms treaty the U.S. observes is New START, a pact with Russia negotiated under the Obama administration. That treaty cut the number of deployed nuclear warheads that each side can have by more than half, to 1,550. Former President Trump was planning to let the treaty expire this month. But just after taking office, President Biden agreed with Russian President Vladimir Putin to extend the treaty for five more years. Biden also will try to revive the Iran nuclear deal.

What is Iran's capability?
Israeli intelligence says that the assassination of Iran's top nuclear scientist in November set Iran's nuclear program back, and that it would need two years to build a nuclear weapon. In the early 2000s, the International Atomic Energy Agency discovered that Iran had been cheating on the NPT with a clandestine program to enrich uranium. Under the 2015 treaty negotiated by the Obama administration, Iran agreed to radically slash its stockpile of uranium and limit the number of centrifuges that it can use for enrichment. But since the Trump administration pulled out of the deal in 2018 and hit Iran with new sanctions, Iran has resumed production of 20 percent enriched uranium, getting nine-tenths of the way toward weapons-grade fuel.

What happens if Iran goes nuclear?
It would set off a chain of proliferation. Saudi Arabia, Iran's enemy, has said it would seek nukes if Iran got them, and Turkey and Egypt could follow. The threat from North Korea, meanwhile, is alarming to Japan and South Korea, where factions have argued for the development of their own nuclear weapons as deterrents. Since it first tested a nuclear weapon in 2006, North Korea has built dozens of bombs and hundreds of missiles, and it now has intercontinental ballistic missiles that can reach anywhere — including the continental United States. Our allies are now wondering, says Ivo Daalder of the Chicago Council on Global Affairs, "Will you sacrifice us for you? Will you save Seattle at the price of Seoul?" The more nuclear weapons there are in the world, of course, the more likely it is that one could be fired by accident or fall into terrorist hands.

What comes next?
The next nuclear summit — the NPT review conference held every five years — takes place in August. That will be a chance for the Biden administration to reassure allies and to open negotiations with rising power China. China is planning to double its arsenal to 200 warheads over the next decade, and it has been pouring money into new missile designs. Adm. Charles Richard, head of the U.S. strategic command, says China will soon be a nuclear peer of the U.S., just as Russia is. "For the first time ever, the U.S. is going to face two peer-capable nuclear competitors who are different, who you have to deter differently," he said. "We have never faced that situation before."

The trouble with missile defense
Missile defense is a system designed to shoot down incoming nuclear missiles before they hit. But if a country can shoot down, say, 100 enemy missiles, the enemy has an incentive to fire 200 to overwhelm the defense, leading to an offensive and defensive arms race. So in their arms control treaty, the U.S. and Soviets banned most missile defenses, relying instead on deterrence — the threat of mutual assured destruction. The U.S. pulled out of that pact in 2002, saying it needed the ability to defend against a launch by a terrorist or a rogue state such as Iran or North Korea. Since then, it has deployed defense systems in South Korea and sold anti-ballistic Patriot missiles to more than a dozen countries. The danger with missile defense is that if a country believes it can reliably defend itself against retaliatory nukes, it loses the deterrence of conducting its own first strike. But so far, despite billions in expenditures, missile defense is more of a fantasy than a reality. Patriot missiles failed to knock down most missiles fired by enemies in the Saudi-Houthi conflict and the 1991 Gulf War. In fact, says arms control expert Jeffrey Lewis, there is no evidence that a Patriot "has ever intercepted a long-range ballistic missile in combat."

This article was first published in the latest issue of The Week magazine. If you want to read more like it, you can try six risk-free issues of the magazine here.