The attack by Taliban fighters this week on a major Pakistani air base where nuclear weapons allegedly were stored offered a dramatic example of what the U.S. fears most about its unstable, nuclear-armed ally. Though Pakistan claimed its forces repelled the attackers and denied that nuclear weapons were even present on the site, the incident inevitably revived long-standing U.S. concerns that terrorists could get their hands on a weapon of mass destruction.

The attack on the Minhas air force base and aeronautical college in Kamra, 37 miles north of Islamabad, was carried out early Thursday morning by gunman armed with automatic weapons and rocket-propelled grenades. Some of the attackers reportedly were wearing military uniforms over their suicide vests. In a two-hour firefight with soldiers on the base, nine of the attackers and one Pakistani soldier were killed, and an airplane hanger and a warplane were damaged. The commander of the base was among those wounded in the battle.

The Taliban quickly claimed responsibility for the attack, which its spokesman said was launched in response to an impending government sweep of the lawless tribal region of North Waziristan. Pakistan is reported to be preparing a major offensive against militants in the area, something its American allies have long been urging. Afghan insurgents based in Pakistan such as the Haqqani network have long used the area to stage attacks against U.S. and NATO forces just across the border.

But the fact that a heavily guarded government military installation like the one at Minhas could be breached by a relatively small group of militants served to underscore Pakistan's continuing vulnerability to its own homegrown insurgency. For years Pakistan's Inter-Services Intelligenceagency helped organize and arm Islamic militant groups in Afghanistan and Kashmir as proxies in its covert war with rival India. That rivalry has also driven Pakistan to build up a nuclear arsenal whose size — thought to be around 100 warheads — now far exceeds any rational calculation of what is needed for the purpose of deterrence.

Armed to the teeth against India but vulnerable at home to an Islamic insurgency that seems unusually adept at infiltrating military and government institutions, the risk of a nuke going missing may be higher in Pakistan than anywhere else in the world today. Pakistani officials may insist that its weapons were never in jeopardy — aside from the plane that was damaged — and that even if the insurgents had managed to somehow make off with a weapon they still wouldn't have the launch codes and keys needed to detonate it. But it's small comfort when the best one can say is that bad as things were, they could have been worse.

We may never know how close the attackers at Minhas came to actually stealing a weapon, or even whether that was their intention. But the mere fact that anal-Qaida-affiliated group got past the fence around a place where weapons terrorists have dreamed of possessing for years allegedly were stored should be enough to keep U.S. policymakers tossing their sleep at night.