The Bulletin of Atomic Scientists examines the grim possibility of nuclear terrorism, noting that the failure to prevent 9/11 itself stemmed from a failure of imagination: “A similar failure of imagination leads many today to discount the risk of a nuclear 9/11.”
How great a risk? Risk equals probability times consequences. During the Cold War, strategists understood that even the slight possibility of a nuclear war that could kill every American made it imperative to do everything possible to avoid nuclear conflict. Similarly, the magnitude of the consequences of even a single nuclear bomb exploding in just one U.S. city swamps differences in judgments about the likelihood of such an attack. A terrorist armed with one nuclear bomb could murder a million people–killing in one day twice as many American souls as died in both World Wars combined.
On a normal workday, half a million people crowd the area within a half-mile radius of New York City’s Times Square. If terrorists detonated a 10-kiloton nuclear weapon in the heart of midtown Manhattan, the blast would kill them all instantly. Hundreds of thousands of others would die from collapsing buildings, fire, and fallout in the hours and days thereafter.
The blast would instantly vaporize Times Square, Grand Central Terminal, and every other structure within half a mile of the point of detonation. Buildings three-quarters of a mile from ground zero would be fractured husks.
Lest this seem too hypothetical, recall an actual incident that occurred in New York City one month to the day after the 9/11 attacks on the World Trade Center and Pentagon. A CIA agent, code-named Dragonfire, reported that Al Qaeda had acquired a live nuclear weapon produced by the former Soviet Union and had successfully smuggled it into New York City.  A top-secret Nuclear Emergency Support Team was dispatched to the city. Under a cloak of secrecy that excluded even Mayor Rudolph Giuliani, these nuclear ninjas searched for the 10-kiloton bomb whose blast could have obliterated a significant portion of Manhattan. Fortunately, Dragonfire’s report turned out to be a false alarm. But the central takeaway from the Dragonfire case is this: The U.S. government had no grounds in science or in logic to dismiss the warning.
A nuclear terrorist attack on the United States would have catastrophic consequences even for other countries. After the nuclear detonation, the immediate reaction would be to block all entry points to prevent another bomb from reaching its target, resulting in the disruption of the global “just-in-time” flow of goods and raw materials. Vital markets for international products would disappear, and closely linked financial markets would crash. Researchers at RAND, a U.S.-government-funded think tank, estimated that a nuclear explosion at the Port of Long Beach in California would cause immediate indirect costs worldwide of more than $3 trillion and that shutting down U.S. ports would cut world trade by 10 percent. 
The negative economic repercussions would reverberate well beyond the developed world. As U.N. Secretary-General Kofi Annan has warned, “Were a nuclear terrorist attack to occur, it would cause not only widespread death and destruction, but would stagger the world economy and thrust tens of millions of people into dire poverty.”  …
How could terrorists deliver a nuclear weapon to its target? Two plausible methods would be to “follow the golf clubs” or “follow the drugs.”
Imagine a woman who lives in Tokyo wants to play golf at Pebble Beach, but prefers to avoid the hassle of carrying her clubs through U.S. customs. How would she get her clubs to the resort? She would call a freight forwarder, provide a plausible description of the contents of her shipment, and have her golf bag picked up at her home. The clubs would travel by ship from Tokyo to the Port of Oakland in California and then by truck to the golf course. The chance of anyone inspecting her bag between her house and the links is less than 3 percent.
If that seems too risky, terrorists might “follow the drugs,” tons of which find their way to U.S. cities every day. The illicit economy for narcotics and illegal immigrants has built up a vast infrastructure that terrorists could exploit. As Albert Carnesale, an arms control expert, has noted, no one should doubt the ability of terrorists to bring a nuclear weapon to New York: They could simply hide it in a bale of marijuana, which we know comes to all global cities.
In sum, my best judgment is that based on current trends, a nuclear terrorist attack on the United States is more likely than not in the decade ahead. Developments in Iraq, Iran, and North Korea leave Americans more vulnerable to a nuclear 9/11 today than we were five years ago. Former Defense Secretary William Perry has said that he thinks that I underestimate the risk. In the judgment of most people in the national security community, including former Sen. Sam Nunn, the risk of a terrorist detonating a nuclear bomb on U.S. soil is higher today than was the risk of nuclear war at the most dangerous moments in the Cold War. Reviewing the evidence, Warren Buffett, the world’s most successful investor and a legendary oddsmaker in pricing insurance policies for unlikely but catastrophic events like earthquakes, has concluded: “It’s inevitable. I don’t see any way that it won’t happen.”