A little more than 60 miles from Brussels Airport, Kleine Brogel Air Base stands as one of six overseas repositories in the world where the United States still stores nuclear weapons. The existence of the bombs is officially neither confirmed nor denied, but it has been well-known for decades.

Yet the presence of these weapons — an estimated 20 American B61 nuclear bombs to be carried and delivered by the Belgian Air Force's dwindling inventory of F-16 fighter jets — did not come up in the news coverage following the Islamic State (IS) bombings last week in Brussels, or in the run-up to President Barack Obama's fourth and last Nuclear Security Summit, being held this week.

Nor was Kleine Brogel mentioned in reports about the shooting death, days after the bombings, of a security guard who worked at a Belgian nuclear facility, or in stories about vulnerabilities at Belgiam's nuclear facilities and power plants. In a prominent editorial entitled "Keeping Nuclear Weapons From Terrorists," the New York Times didn't mention that US nuclear weapons are stored in Belgium while arguing that "even if the chances are small that terrorists will acquire a nuclear weapon," the potential consequences are so devastating, we should plug any "possible security gaps."

To ensure those gaps are plugged, hundreds of millions of dollars has been spent by the US on security in the two decades since the breakup of the Soviet Union. Belgium's bombs are stored inside specially constructed armored vaults underneath protective aircraft shelters that are inside a secured storage site exclusively used for nuclear weapons that is inside a military base.

According to Hans Kristensen, the director of the nuclear information project of the Federation of American Scientists, nuclear weapons have been in Belgium since November 1963, when they arrived under a Top Secret agreement codenamed Pine Cone that was never seen or approved by the Belgian Parliament. A special American unit, the 130-person strong 701st Munitions Support Squadron, the majority of whom are Air Force security police, maintains 11 vaults at Kleine Brogel, along with specialized assets, aircraft, and bombs valued at more than $6.6 billion dollars, according to the resume of one veteran of the base.

Conventional wisdom is that the nuclear weapons in Belgium, as well as the other four European countries — the Netherlands, Germany, Italy, and Turkey — where they are stored today (there are two storage sites in Italy) tie together the Atlantic alliance in unique ways. The nuclear weapons are the ultimate American guarantee to Europe, and they're also a kind of US-European bargain: Only the US president can approve their use, but the host nations have to approve the bombs leaving the country. Thus the weapons can serve as a deterrent while also being virtually unusable.

The figure of 150 nuclear weapons in Europe today is dwarfed by the number that were stored on European soil at their peak — 7,300 in 1971. Then, there were artillery and missile warheads of a staggering variety, a nuclear bazooka, and even backpack nuclear weapons. Ships and submarines routinely carried nukes into the Baltic and Black seas to the Soviet Union's doorstep. Policy was driven by perceptions of overwhelming conventional military threat and the doctrines of deterrence, with arsenals growing ever more capable.

Then the rise of European terrorism — or at least, the threat of it — in Germany, Italy, and Greece changed everything. A 1974 Senate investigation discovered that there were more than 750 separate sites for the storage of nuclear weapons, some with embarrassingly rudimentary physical security. Even if there were a Soviet invasion of Europe, there were so many nuclear weapons, and so many were so close to the border, that their locations would influence the likelihood of the use of the weapons rather than a political decision.

One by one, weapons were returned to the United States and retired, and European storage sites were closed. By the late 1980s, the first formal European nuclear treaty was signed, and by the end of the Cold War a few years later, only US bombs to be delivered by aircraft remained in Europe. Soviet nuclear weapons had even been removed from Eastern Europe. Security was tightened.

Public disquiet also played an important role: European peace movements in the 1980s provoked tightened security and greater secrecy, but they also prompted European parliaments and news media to truly examine, for the first time, the makeup and logic of the nuclear arsenal. In 1981, when I first revealed the exact locations of all nuclear weapons stored in Europe, then US Secretary of State Alexander Haig told his legal office that he wanted me in jail. Months of negotiations followed and a deal was worked out: If I could prove that I didn't access classified information in figuring out where the nuclear weapons were — I was fresh out of Army intelligence — the government wouldn't bring charges. Armed with boxes of index cards and military manuals and telephone books, I made a convincing presentation in a State Department auditorium.

It was a turning point: If the Soviet Union knew where the nuclear weapons were and the goal was physical security, secrecy wasn't and couldn't be the determinant. Thus started the age of nuclear transparency. Public pressure led to nuclear weapons being withdrawn from a half-dozen countries. Greenpeace's Nuclear Free Seas campaign, which dogged US and Russian nuclear-armed ships while there were countless local protests in harbors around the world, became such a nuisance that the first Bush administration decided to remove all tactical nuclear weapons from naval ships. American nuclear weapons were even withdrawn from South Korea, where they were facing down the North Koreans. But the NATO bombs in Belgium and elsewhere in Western Europe survived, conventional wisdom in the new age of terrorism being that they were completely secure.

Today, the 150 bombs evade public attention to the extent that a post–terror attack nuclear scare in Belgium can occur without the bombs even being mentioned. And in terms of physical security, with five layers of control, every electronic gizmo known to the world of security, and 300 US and Belgian full-time guards, the bombs are safe.

But they are not as unmoving (or important) as nuclear advocates might make you believe. First of all, in terms of deterrence, if nuclear weapons could be removed from the Korean Peninsula, certainly they don't need to be physically present in Europe. Second, there is no earthly reason why nuclear weapons should be at Kleine Brogel, certainly not if the country is under threat, certainly not if they are attractive target for terrorism or there is the slightest risk. After all, other NATO nuclear partners have denuclearized. In 2001, the last nuclear weapons were withdrawn from Greece. US nuclear weapons were even withdrawn from Britain in 2008.

"A Tunisian-born national was sentenced to 10 years in prison for plotting to bomb Kleine Brogel in 2003," Kristensen says. "Suspected terrorists have had their eye on one of the Italian bases, and the largest nuclear stockpile in Europe is in the middle of an armed civil uprising in Turkey less than 70 miles from war-torn Syria. Is this really a safe place to store nuclear weapons?"

Inside the halls of NATO, the future of nuclear weapons is a simmering political issue, some of the nuclear faithful and their new Eastern European allies arguing that readiness should be beefed up, and that nuclear weapons should be used more for "signaling" against a militaristic Russia. At the NATO Summit coming up in Warsaw in July, the possibility of a new "strategic concept" involving nuclear weapons is rumored to be on the agenda.

The nuclear weapons still deployed in southeast Turkey, about 300 miles from the Islamic State capital of Raqqa, Syria, are an absurd anachronism, and according to Kristensen, millions of dollars are now being spent to improve the security of the US-Turkish airbase at Incirlik.

The US says that it spends about $100 million annually to maintain nuclear weapons in Europe, not counting the cost of the estimated 3,000 people directly involved in security, maintenance, and command and control. The argument is that the price is relatively small. Post-Paris and post-Brussels Europe will no doubt look for more money for intelligence and law enforcement. Pressure is even mounting to increase military spending. NATO's nuclear apparatus would be a good place to start to save money and increase safety. Even Belgium would benefit, with tens of millions of euros freed up to spend on, well, anything else.

Follow William M. Arkin on Twitter: @warkin