B-61 nuclear bombs, the same model as those stored by the U.S. at airbases in various NATO countries, often under lax safeguards. PHOTOGRAPH BY PHIL SCHMITTEN /UNITED STATES DEPARTMENT OF DEFENSE

Among the many questions still unanswered following Friday’s coup attempt in Turkey is one that has national-security implications for the United States and for the rest of the world: How secure are the American hydrogen bombs stored at a Turkish airbase?

The Incirlik Airbase, in southeast Turkey, houses NATO’s largest nuclear-weapons storage facility. On Saturday morning, the American Embassy in Ankara issued an “Emergency Message for U.S. Citizens,” warning that power had been cut to Incirlik and that “local authorities are denying movements on to and off of” the base. Incirlik was forced to rely on backup generators; U.S. Air Force planes stationed there were prohibited from taking off or landing; and the security-threat level was raised to FPCON Delta, the highest state of alert, declared when a terrorist attack has occurred or may be imminent. On Sunday, the base commander, General Bekir Ercan Van, and nine other Turkish officers at Incirlik were detained for allegedly supporting the coup. As of this writing, American flights have resumed at the base, but the power is still cut off.

According to Hans M. Kristensen, the director of the Nuclear Information Project at the Federation of American Scientists, underground vaults at Incirlik hold about fifty B-61 hydrogen bombs—more than twenty-five per cent of the nuclear weapons in the NATO stockpile. The nuclear yield of the B-61 can be adjusted to suit a particular mission. The bomb that destroyed Hiroshima had an explosive force equivalent to about fifteen kilotons of TNT. In comparison, the “dial-a-yield” of the B-61 bombs at Incirlik can be adjusted from 0.3 kilotons to as many as a hundred and seventy kilotons.

Incirlik was built by the U.S. Army Corps of Engineers in the wake of the Second World War; when Turkey joined NATO, in 1952, it became a crucial American base during the Cold War. With a flight time of about an hour to the Soviet Union, the base hosted American fighters, bombers, tankers, and U-2 spy planes. And, like many NATO bases, it stored American nuclear weapons. NATO strategy was dependent on nuclear weapons as a counterbalance to the perceived superiority of Soviet conventional forces. The threat of a nuclear attack, it was assumed, would deter Soviet tanks from rolling into NATO territory. And granting NATO countries access to nuclear weapons would strengthen the alliance, providing tangible evidence that the United States would risk a nuclear war for NATO’s defense.

By the mid-nineteen-sixties, more than seven thousand American nuclear weapons were deployed in Western Europe, Greece, and Turkey. They came in all sizes, shapes, and yields: nuclear warheads, bombs, land mines, depth charges, artillery shells, even small nuclear projectiles that could be fired from a recoilless rifle. The weapons were technically in the custody of U.S. officers, ready to be handed over for use in wartime by NATO personnel. But custody of the weapons was not the same as control of them. A delegation of U.S. senators visiting Europe in 1960 was shocked to find hydrogen bombs loaded onto German planes that were on alert and crewed by German pilots; thermonuclear warheads atop missiles manned by Italian crews; nuclear weapons guarded and transported by “non-Americans with non-American vehicles.” The theft or use of these weapons by NATO allies became a grave concern. “The prime loyalty of the guards, of course, is to their own nation, and not to the U.S.,” the Senate delegation warned in a classified report.

Two years later, during the Cuban Missile Crisis, Secretary of Defense Robert McNamara worried that Turkish officers might try to fire some of NATO’s nuclear missiles at the Soviet Union without permission—and ordered American custodians to sabotage the missiles, somehow, if anyone tried to launch them. Coded switches were subsequently placed inside NATO’s hydrogen bombs. These switches, known as Permissive Action Links (PALs), were designed to hinder unauthorized use of the weapons; the bombs wouldn’t detonate if the operator didn’t enter the right code. But PALs could be circumvented by someone with the proper technical skills. When two NATO allies, Greece and Turkey, were on the cusp of war in 1974, the United States secretly removed all of NATO’s nuclear weapons from Greece and cut the arming wires of every nuclear weapon stored in Turkey, rendering them inoperable.

Thanks largely to stockpile reductions during the Administrations of President George H. W. Bush and President George W. Bush, the United States now has about a hundred and eighty nuclear weapons deployed with NATO, all of them B-61 bombs. In addition to Incirlik, the weapons are stored at bases in Germany, the Netherlands, Belgium, and Italy. Today, the symbolism of these bombs is far more important than their military utility; missiles carrying nuclear warheads reach targets much faster, more reliably, and with much greater accuracy. The advocates of retaining nuclear weapons in NATO argue that the B-61 bombs demonstrate America’s enduring commitment to the alliance, intimidate Russia, and discourage NATO members from developing their own hydrogen bombs. Opponents of the weapons, like Frank-Walter Steinmeier, the German foreign minister, consider them “absolutely senseless”—and an inviting target for terrorists.

With a few hours and the right tools and training, you could open one of NATO’s nuclear-weapons storage vaults, remove a weapon, and bypass the PAL inside it. Within seconds, you could place an explosive device on top of a storage vault, destroy the weapon, and release a lethal radioactive cloud. NATO’s hydrogen bombs are still guarded by the troops of their host countries. In 2010, peace activists climbed over a fence at the Kleine Brogel Airbase, in Belgium, cut through a second fence, entered a hardened shelter containing nuclear-weapons vaults, placed anti-nuclear stickers on the walls, wandered the base for an hour, and posted a video of the intrusion on YouTube. The video showed that the Belgian soldier who finally confronted them was carrying an unloaded rifle.

Security concerns at Incirlik Airbase recently prompted a major upgrade of the perimeter fence that surrounds its nuclear-weapons storage area. Incirlik is about seventy miles from the Syrian border, and since October American aircraft and drones based there have been attacking ISIS forces. Its proximity to rebel-controlled areas in Syria and the rash of terrorist acts in Turkey led the Pentagon, a few months ago, to issue an “ordered departure” of all the family members of American troops at Incirlik. They were asked to leave immediately. About two thousand U.S. military personnel remain stationed there. Although Incirlik probably has more nuclear weapons than any other NATO base, it does not have any American or Turkish aircraft equipped to deliver them. The bombs simply sit at the base, underground, waiting to be used or misused.