In recent years, American and Israeli fears of nuclear proliferation have focused on Iran. The consequences of an Iranian bomb could be grave indeed: a chain reaction of nuclear armament among Arab countries, some of whom are threatened by, or may collaborate with, jihadists.

It is unlikely, however, that Iran would start a nuclear war: its regime has a return address, and Israel could annihilate them. That is why nuclear terrorism by non-state actors like Al Qaeda is the West’s ultimate nightmare and why Pakistan, not Iran, is the most dangerous place on earth.

Imagine this: Three jihadist groups in Pakistan—Al Qaeda, the Taliban and Lashkar-e-Taiba (“LET”)—forge an operational alliance to steal a nuclear bomb from the Pakistani arsenal in order to destroy a major Western city. Pursuant to the plan, LET—which carried out the Mumbai attacks—destroys the Taj Mahal and attacks the Indian Parliament, precipitating a state of nuclear alert between India and Pakistan, whose intelligence agency is the chief sponsor of LET.

When a Pakistani convoy moves a bomb from its secret storage facility to an Air Force base near the border, a group of Pakistani Taliban—directed by Al Qaeda and tipped off by a military insider—attacks the convoy and steals the bomb. From there, Al Qaeda has several routes for smuggling the bomb to America, Europe, or Israel.

This is not a Bondian fantasy. What is so frightening about this scenario is its realism: every detail is of grave concern to the national defense and intelligence communities. But almost as disturbing is how little most Americans know about this threat.

There is no country with more active terrorists than Pakistan, and few with more nuclear weapons. The spur for nuclear armament is Pakistan’s bitter rivalry with India, focused on the violent sixty-year-old dispute over Kashmir. The unintended consequences could be lethal: a jihadist capture of nuclear weapons or materials for use against the West.

This could happen in several different ways: the clandestine acquisition of nuclear materials; seizure of a nuclear facility by a rogue military officer; a jihadist takeover of the Pakistan government; and the theft of a nuclear weapon.

Underlying these fears are serious questions about the security of the Pakistani arsenal. We don’t know where all the weapons are stored. The people who do—the military and the Pakistani intelligence service, the ISI—include highly-placed jihadist sympathizers, typified by a former head of the ISI who has said, “The same nuclear capacity that can destroy Madras, India, can destroy Tel Aviv.” The weapons themselves may lack American-style security systems—which operate like a sophisticated ATM—to prevent accidental or unauthorized use. The tension between Pakistan and India poses the constant threat of a nuclear alert. And nuclear weapons are never less secure than when they are moved from one site to another.

Al Qaeda has long been obsessed with nuclear weapons, and Pakistan has always been its focus. Just before 9/11, Bin Laden met in Afghanistan with a Pakistani nuclear scientist and an engineer, drawing up specifications for an Al Qaeda bomb. And after 9/11, Bin Laden announced Al Qaeda's intention to kill four million Americans to "balance" the Muslim deaths he attributes to the United States and Israel and issued a fatwa calling for the use of nuclear arms against the West.

We are certainly vulnerable to this. A Pakistani bomb carries enough HEU to destroy New York, but can travel in a container the size of a coffin. Its total weight is between 200 to 300 pounds, which means that a few men could put it in a van, truck, boat, cargo container, or private plane.

Such a weapon could easily be smuggled through the ports in Long Beach or New York, where we inspect roughly 2% of all cargo containers. From there, a small aircraft could deliver a nuclear weapon to any city in America.

For example, let's take Washington, D.C. In theory, we’ve got a fifteen mile no-fly zone around the capital, enforced by surface-to-air missiles and jets at Andrews Air Force Base on a five-minute alert. But thousands of aircraft fly within fifteen miles of the White House—if one crosses the line going 300 miles an hour, five minutes won’t be enough. Though the government won’t say so, multiple planes fly over the capital every year, and we don’t spot half of them until it’s over.

There’s a more than fair chance that Al Qaeda could turn the White House into the epicenter of a nuclear blast. A strike against Washington, D.C. or New York could be economically, politically, and psychologically shattering. Terrified of Al Qaeda, Americans would be thrown into a panic while they wait for the next city to disappear. This would threaten our own belief in our government, our system of civil liberties, or even our future as a democracy.

As for Israel itself, the impact could well be fatal. A recent poll revealed that one-fourth of Israelis would consider emigrating if Iran develops a bomb.

Imagine, then, the impact of a strike on Tel Aviv that annihilates hundreds of thousands of Israelis, destroying the heart of their infrastructure, their economy, and, most fundamental, the belief that they can survive as a nation. One likelihood is that Israel would become a Masada state, populated by a cadre of religious fanatics prepared to watch their families die rather than yield an inch of their atomic wasteland. The result would be unspeakably sad—the end of Israel as we know it.

So how realistic is a scenario in which Al Qaeda links with other jihadists to steal a nuclear weapon from Pakistan? Very. But to grasp this, one must understand the links between jihadist groups, and the ties between these groups and the most powerful forces within Pakistan—the military who controls the nuclear arsenal and, in particular, the ISI.

The ISI is at the heart of Pakistani jihadism. It helped create the Taliban to fight the Russians in Afghanistan, and introduced its leaders to bin Laden. It created LET to fight a guerilla war against India in Kashmir. The military, the ISI and LET all recruit among the Punjabi, Pakistan’s dominant ethnic group, creating familial ties between all three. With the ISI’s protection, LET—despite Mumbai—trains hundreds of jihadists every year. And the ISI is so marbled with jihadist sympathizers that joint operations with the CIA are often next to impossible—witness its current demand that we withdraw hundreds of intelligence and Special Operations personnel, a fresh expression of antipathy that weakens our intelligence, strengthens the militants and exacerbates Pakistan’s nuclear danger.

As for the Taliban and LET, according to leading experts they are now allied with Al Qaeda against the West and planning further attacks on America and its allies at home and abroad. Al Qaeda and the Taliban have safe havens in Pakistan—including, most believe, Bin Laden—and Benazir Bhutto’s murder was most likely their joint operation.

As for LET, Al Qaeda helped to fund it, and after 9/11 some of its leaders took refuge in LET safe houses.

U.S. intelligence officials believe that LET’s targets now include America, Israel, and Europe. And all three groups are Sunni and, increasingly, share Al Qaeda’s goal of jihad against the West.

So what do we do about Pakistan itself? Cutting off aid would only destabilize the country further. Instead, we must pursue the undramatic but essential work that any hard situation requires: engagement, patience, consistency, prudent intelligence work under hard circumstances, and smart diplomacy. We must engage civilian leaders, encourage the development of civil institutions, fund greater aid earmarked for universal education, increase military-to-military contact; and quietly work for a rapprochement with Indian and Pakistani acquiescence in the international regime governing nuclear proliferation.

Obviously, there are no simple answers. But protecting ourselves against disaster requires the willingness to look ahead for decades, not weeks or months or even years. The foreign policy of a great nation in a nuclear age does not shy from complex challenges—it embraces them.

Richard North Patterson is the author of eighteen bestselling and critically acclaimed novels. His latest book "The Devil’s Light" (Scribner) will be published in May.