The story goes that a weakling gets sand kicked in his face on the beach. He then joins a gym, pumps iron and returns to the beach, where all he has to do is flex his new muscles, and he is left alone.

That, it would seem, is one lesson of nuclear weapons. Small countries might be left alone if they had nuclear weapons, which would seem to be the case with North Korea: unloved but uninvaded.

In the case of Iran, which has sought a nuclear weapon for a long time, the fear was that it would do more than discourage aggression: It would move aggressively against Israel.

It also raises the question: Would Iran have been attacked by Israel and the United States if it already had a nuclear weapon?

Israel is a small country with 10.2 million people, and a land mass equivalent to New Jersey. By contrast, Iran has more than 90 million people and a land mass more than twice the size of Texas. It is a big place to sustain an attack and to hide men and materiel, to say nothing of secret weapons development centers.

Israel and the United States have attacked Iran, but when it ends, what kind of peace can they expect?

The Iran war — and the one by Russia against Ukraine — is making the case for smaller nations to get a nuclear weapon of their own.

Ukraine voluntarily gave up its weapons — the third-largest nuclear arsenal after Russia and the United States — after the collapse of the Soviet Union. In exchange, the United States, Britain and Russia would guarantee Ukraine’s security in a 1994 agreement known as the Budapest Memorandum.

Would the Russians have invaded if Ukraine still had its weapons?

The lesson of that war is clear: You could be attacked for assorted reasons, but if you have a nuclear weapon, that likelihood is diminished.

The case in point is North Korea and its oppressive and dangerous regime. It is a threat to its neighbors and has an asocial stance internationally. Yet, the United States, South Korea and Japan have never proposed attacking it.

Over the years, there have been many studies among these allies as to how its communist regime might be brought down with force. The fear that the North Koreans would launch a nuclear attack on Seoul, Tokyo or even the West Coast of the United States has always been uppermost in the planning. No American president has been asked to approve a takedown of the country. It is too dangerous.

Nuclear proliferation is again an issue that the nations of the world need to heed. Not only is it frighteningly real, but it may be easier than ever.

In a severe report last July, Alan J. Kuperman, coordinator of the Nuclear Proliferation Prevention Project at the University of Texas at Austin, raised the alarm that the drive for small modular reactors here and around the world would increase the chances of nuclear proliferation, and the Bulletin of the Atomic Scientists moved the “Doomsday Clock” forward.

The original nuclear weapons states were the United States, Russia, France and Britain. The world was shaken up when China joined the club in 1964, and again when India did so in 1974, Pakistan in 1998, and North Korea — the most worrying of all — in 2006.

Long term, more weapons make the world more dangerous, more subject to crazed governments and autocrats.

Concern about nuclear proliferation dominated U.S. nuclear policy for decades and was at a peak after the Chinese advance, and another peak when Pakistan became the first Muslim country to get a nuclear bomb.

President Jimmy Carter moved aggressively to avoid the risk that the United States could inadvertently contribute to proliferation. He cancelled the reprocessing of nuclear fuel, the breeder reactor program, and discouraged some new reactor ideas, which are, again, being developed.

Now, nuclear weapons are being considered by Poland, Saudi Arabia, Japan and South Korea, according to sources in the nuclear establishment.

Not only are nations looking again to nuclear weapons for their own defense, but designing and engineering them may also be easier with artificial intelligence, which can perform thousands of calculations instantly.

Edward Teller, father of the hydrogen bomb, once told me that calculations were the root of the whole weapons enterprise. In 1955, in records that are now declassified, he urged the development of “better computing machines” for nuclear weapons development.

Nuclear proliferation is a cause for deep alarm as mankind enters a new epoch where old treaties lose their meaning and where the vulnerable are seeking defense against the hegemons.

As Daniel Patrick Moynihan, the late, great senator from New York and former U.N. ambassador, said, “The world is a dangerous place.”

The war in the Middle East is making it more so.